Why does sizeof(x++) not increment x?

ゞ浴缸里的玫瑰 2022-05-24 06:23 97阅读 0赞

原帖: [http://stackoverflow.com/questions/8225776/why-does-sizeofx-not-increment-x][http_stackoverflow.com_questions_8225776_why-does-sizeofx-not-increment-x]

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Here is the code compiled in dev c++ windows:

#include <stdio.h> int main() {     int x = 5;     printf("%d and ", sizeof(x++)); // note 1     printf("%d\n", x); // note 2     return 0; }

I expect `x` to be 6 after executing note 1. However, the output is:

4 and 5

Can anyone explain why `x` does not increment after note 1?

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From the [C99 Standard][] (the emphasis is mine)

> 6.5.3.4/2
> 
> The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

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`sizeof` is a compile-time operator, so at the time of compilation `sizeof` and its operand get replaced by the result value. The operand is not evaluated (except when it is a variable length array) at all; only the type of the result matters.

short func(short x) {  // this function never gets called !!    printf("%d", x);    // this print never happens    return x; } int main() {    printf("%d", sizeof(func(3))); // all that matters to sizeof is the                                   // return type of the function.    return 0; }

Output:

as `short` occupies 2 bytes on my machine.

Changing the return type of the function to `double`:

double func(short x) { // rest all same

will give `8` as output.

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`sizeof(foo)` tries really hard to discover the size of an expression at compile time:

6.5.3.4:

> The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

In short: variable length arrays, run at runtime. (Note: [Variable Length Arrays][] are a specific feature -- not arrays allocated with `malloc(3)`.) Otherwise, only the type of the expression is computed, and that at compile time.

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`sizeof` is a compile-time builtin operator and is not a function. This becomes very clear in the cases you can use it without the parenthesis:

(sizeof x)  //this also works

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The execution cannot happen during compilation. So  `++i` / `i++`  will not happen. Also  `sizeof(foo())`  will not execute the function but return correct type.

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As the operand of `sizeof` operator is not evaluated, you can do this:

int f(); //no definition, which means we cannot call it int main(void) {         printf("%d", sizeof(f()) );  //no linker error         return 0; }

Online demo : [http://ideone.com/S8e2Y][http_ideone.com_S8e2Y]

That is, you don't need define the function `f` if it is used in `sizeof` only. This technique is mostly used in C++ template metaprogramming, as even in C++, the operand of `sizeof` is not evaluated.

Why does this work? It works because the `sizeof` operator doesn't operate on value, instead it operates ontype of the expression. So when you write `sizeof(f())`, it operates on the type of the expression `f()`, and which is nothing but the return type of the function `f`. The return type is always same, no matter what value the function would return if it actually executes.

In C++, you can even this:

struct A {   A(); //no definition, which means we cannot create instance!   int f(); //no definition, which means we cannot call it }; int main() {         std::cout << sizeof(A().f())<< std::endl;         return 0; }

Yet it looks like, in `sizeof`, I'm first creating an instance of `A`, by writing `A()`, and then calling the function `f`on the instance, by writing `A().f()`, but no such thing happens.

Demo : [http://ideone.com/egPMi][http_ideone.com_egPMi]

Here is another topic which explains some other interesting properties of `sizeof`:

*  [sizeof taking two arguments][]

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原帖: [http://stackoverflow.com/questions/8225776/why-does-sizeofx-not-increment-x][http_stackoverflow.com_questions_8225776_why-does-sizeofx-not-increment-x]

===============================================================================================================================

Here is the code compiled in dev c++ windows:

#include <stdio.h> int main() {     int x = 5;     printf("%d and ", sizeof(x++)); // note 1     printf("%d\n", x); // note 2     return 0; }

I expect `x` to be 6 after executing note 1. However, the output is:

4 and 5

Can anyone explain why `x` does not increment after note 1?

===============================================================================================================================

From the [C99 Standard][] (the emphasis is mine)

===============================================================================================================================

Output:

as `short` occupies 2 bytes on my machine.

Changing the return type of the function to `double`:

double func(short x) { // rest all same

will give `8` as output.

===============================================================================================================================

`sizeof(foo)` tries really hard to discover the size of an expression at compile time:

6.5.3.4:

===============================================================================================================================

`sizeof` is a compile-time builtin operator and is not a function. This becomes very clear in the cases you can use it without the parenthesis:

(sizeof x)  //this also works

===============================================================================================================================

The execution cannot happen during compilation. So  `++i` / `i++`  will not happen. Also  `sizeof(foo())`  will not execute the function but return correct type.

===============================================================================================================================

As the operand of `sizeof` operator is not evaluated, you can do this:

int f(); //no definition, which means we cannot call it int main(void) {         printf("%d", sizeof(f()) );  //no linker error         return 0; }

Online demo : [http://ideone.com/S8e2Y][http_ideone.com_S8e2Y]

In C++, you can even this:

Yet it looks like, in `sizeof`, I'm first creating an instance of `A`, by writing `A()`, and then calling the function `f`on the instance, by writing `A().f()`, but no such thing happens.

Demo : [http://ideone.com/egPMi][http_ideone.com_egPMi]

Here is another topic which explains some other interesting properties of `sizeof`:

*  [sizeof taking two arguments][]

===============================================================================================================================

[http_stackoverflow.com_questions_8225776_why-does-sizeofx-not-increment-x]: http://stackoverflow.com/questions/8225776/why-does-sizeofx-not-increment-x
[C99 Standard]: http://www.open-std.org/JTC1/sc22/wg14/www/docs/n1256.pdf
[Variable Length Arrays]: http://en.wikipedia.org/wiki/Variable-length_array
[http_ideone.com_S8e2Y]: http://ideone.com/S8e2Y
[http_ideone.com_egPMi]: http://ideone.com/egPMi
[sizeof taking two arguments]: http://stackoverflow.com/questions/6331588/sizeof-taking-two-arguments