410 lines
10 KiB
Markdown
410 lines
10 KiB
Markdown
---
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title: Macros
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TARGET DECK: Obsidian::STEM
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FILE TAGS: c17::macro
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tags:
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- c17
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---
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## Overview
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Macros refer to `#define` directives that specify terms that should be textually replaced by the preprocessor during compilation:
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```c
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#define NAME ...
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```
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For types that don't have literals that describe their constants, we can use **compound literals** on the replacement side of the macro:
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```c
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#define NAME (T){ INIT }
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```
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%%ANKI
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Basic
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What preprocessor directive is used to define macros?
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Back: `#define`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419429-->
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END%%
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%%ANKI
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Basic
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How are compound literals specified in a macro definition, say `MACRO`?
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Back:
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```c
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#define MACRO (T){ INIT }
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```
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419447-->
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END%%
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%%ANKI
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Basic
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What term is used to refer to the replacement side of the following macro?
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```c
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#define MACRO (T){ INIT }
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```
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Back: A compound literal.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419481-->
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END%%
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%%ANKI
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Basic
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What is the difference between the following two lines?
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```c
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#define MACRO (T){ INIT }
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# define MACRO (T){ INIT }
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```
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Back: N/A. They are equivalent.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419485-->
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END%%
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%%ANKI
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Basic
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What is the difference between the following two lines?
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```c
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#define MACRO (T){ INIT }
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#define MACRO(T){ INIT }
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```
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Back: The first defines a compound literal. The latter defines a function-like macro.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419489-->
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END%%
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%%ANKI
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Basic
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What is `T` a reference to in the following compound literal?
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```c
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#define MACRO (T){ INIT }
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```
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Back: A type-specifier.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419492-->
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END%%
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%%ANKI
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Basic
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What is `INIT` a reference to in the following compound literal?
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```c
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#define MACRO (T){ INIT }
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```
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Back: An initializer.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419495-->
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END%%
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%%ANKI
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Basic
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Why aren't compound literals suitable for ICE?
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Back: They are objects, not constants.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419498-->
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END%%
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%%ANKI
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Basic
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How can the following be rewritten so that `MACRO` is an object?
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```c
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#define MACRO 5
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```
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Back:
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```c
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#define MACRO (int){5}
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```
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419500-->
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END%%
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%%ANKI
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Basic
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What is the difference between the following two lines?
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```c
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#define MACRO 5
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#define MACRO (int){5}
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```
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Back: The former is a literal whereas the latter is a compound literal.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419503-->
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END%%
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%%ANKI
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Basic
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Why should compound literals be, generally speaking, `const`-qualified?
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Back: Doing so gives the optimizer more room to generate good binary code.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419506-->
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END%%
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%%ANKI
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Basic
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How do we write macro definitions that span more than one line?
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Back: Ending all but the last line with a `\` character.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419508-->
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END%%
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%%ANKI
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Basic
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Generally speaking, what character should *not* be specified at the end of a macro definition?
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Back: `;`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1727432419511-->
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END%%
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## Operators
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The `#` operator is used to stringify an argument. For example, calling `foo(bar)` in the following yields `"bar"`.
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```c
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#define foo(T) #T
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```
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%%ANKI
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Basic
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In what context is the `#` operator found?
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Back: Macros.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613466-->
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END%%
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%%ANKI
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Basic
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What does the `#` operator do?
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Back: Stringify arguments.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613469-->
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END%%
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%%ANKI
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Basic
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Given the following definition, what is the result of `foo(bar)`?
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```c
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#define foo(T) #T
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```
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Back: `"bar"`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613472-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T) #T
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int main(void) { printf("%s\n", foo(123)); }
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```
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Back: `123`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613475-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T) #T
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int main(void) { printf("%s\n", foo("world")); }
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```
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Back: `"world"` (*with* quotations).
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613477-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T) #T
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int main(void) { printf("%s\n", foo(hello)); }
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```
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Back: `hello`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613480-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, U) #T
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int main(void) { printf("%d\n", foo(123)); }
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```
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Back: N/A. The `foo` function macro invocation returns a string, not an integer.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613483-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, U) #T #U
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int main(void) { printf("%s\n", foo(123, 456)); }
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```
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Back: `123456`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613486-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, U) #T " " #U
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int main(void) { printf("%s\n", foo("123", "456")); }
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```
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Back: `"123" "456"`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613488-->
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END%%
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The `##` operator is used to concatenate tokens. For example, calling `foo(bar)` in the following yields `bar_init`.
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```c
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#define foo(T) T ## _init
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```
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%%ANKI
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Basic
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In what context is the `##` operator found?
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Back: Macros.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613492-->
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END%%
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%%ANKI
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Basic
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What does the `##` operator do?
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Back: Concatenate tokens together.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613495-->
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END%%
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%%ANKI
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Basic
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Given the following definition, what is the result of `foo(bar)`?
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```c
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#define foo(T) T ## _init
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```
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Back: `bar_init`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613498-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, U) #T ## #U
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int main(void) { printf("%s\n", foo(123, 456)); }
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```
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Back: N/A. Pasting formed `"123""456"`, an invalid preprocessing token.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613501-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, U) T ## U
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int main(void) { printf("%s\n", foo("123", "456")); }
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```
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Back: N/A. Pasting formed `"123""456"`, an invalid preprocessing token.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613504-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, U) T ## U
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int main(void) { printf("%s\n", foo(123, 456)); }
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```
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Back: N/A. The `foo` function macro invocation returns an integer, not a string.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733308613507-->
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END%%
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## Variable Argument Lists
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The construct `...` provides an argument list of variable length. This set of arguments is then accessible inside the macro expansion as `__VA_ARGS__`.
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%%ANKI
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Basic
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How is a variable argument list denoted in the macro function declaration?
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Back: As `...`.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733309129384-->
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END%%
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%%ANKI
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Basic
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How is a variable argument list denoted in the macro function expansion?
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Back: As `__VA_ARGS__`.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733309129389-->
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END%%
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%%ANKI
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Cloze
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Within macros, {`...`} corresponds to {`__VA_ARGS__`}.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733309129392-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, ...) #T #__VA_ARGS__
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int main(void) { printf("%s\n", foo(123, 456, 789)); }
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```
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Back: `123456, 789`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733309129395-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, ...) T ## __VA_ARGS__
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int main(void) { printf("%s\n", foo(123, 456, 789)); }
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```
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Back: N/A. The `foo` function macro invocation returns integers, not a string.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733309129398-->
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END%%
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%%ANKI
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Basic
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Given the following translation unit, what is output to `stdout`?
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```c
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#define foo(T, ...) T ## __VA_ARGS__
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int main(void) { printf("%d\n", foo(123, 456, 789)); }
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```
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Back: `123456`
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733309129401-->
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END%%
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## Bibliography
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* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020). |