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I am reading Modern C++ design. It was mentioned about sizeof opeator as following description. Following paragraph is explained from generic programming point of view.

There is a surprising amount of power in sizeof: You can apply sizeof to any expression, no matter how complex, and sizeof returns its size without actually evaluating that expression at runtime. This means that sizeof is aware of overloading, template instantiation, conversion rules—everything that can take part in a C++ expression. In fact, sizeof conceals a complete facility for deducing the type of an expression; eventually, sizeof throws away the expression and returns only the size of its result.

My question is what does author mean sizeof returns its size with out actually evalutating the exression at runtime. And also in last line it was mentioned that sizeof throws away the expression. Request help in understanding these statements, it would be good if it is done with example.

Thanks

Haakon
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venkysmarty
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  • Think of `i++` or function/method calls in an expression; `sizeof` apparently won't execute those while following the types through to reporting the size of a the result. I never knew C++ `sizeof` was so powerful :) thanks! – sarnold Feb 26 '11 at 11:08
  • this post may help you http://stackoverflow.com/questions/671790/how-does-sizeofarray-work – Vivek Goel Feb 26 '11 at 11:12
  • I guess it's why `decltype` was one of the first C++0x available, since the mechanics were already available anyway :) – Matthieu M. Feb 26 '11 at 12:45

5 Answers5

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sizeof is evaluated at compile time: the compiler computes the type of the expression that follows the sizeof operator. This is done once and for all by the compiler, hence the sentence “without actually evaluating that expression at runtime”.

The compiler computes the type, then it is able to deduce the size of the expression from the type, and then, still at compile time, the whole sizeof expression is replaced by the calculated size. So the expression itself does not make it into the executable code. That's what the sentence “sizeof throws away the expression and returns only the size of its result” means.

ChrisJ
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what does author mean sizeof returns its size with out actually evalutating the exression at runtime.

It means that sizeof(1/0) will yield sizeof(int), even though 1/0 would normally abort the program, because division by zero is a runtime error. Also, for any p declared as T* p, sizeof(*p) will yield sizeof(T) no matter what value is stored in p, even if p is dangling or not initialized at all.

fredoverflow
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The following gives you the sizeof of the type that i++ has, which is int (usually an int has 4 or 8 bytes, so it will likely give you value 4 or 8). However, since the expression is not evaluated, no runtime action is done for the expression. 

int i = 0;
sizeof(i++);

Evaluating an expression basically means to execute its side effects (e.g incrementing a variable) or reading values from memory or registers at runtime. So in some sense sizeof "throws away" its operand, since it does not really perform the runtime operation it specifies (the value of i will still be zero).

Johannes Schaub - litb
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The compiler needs to calculate the sizes of types/structs/classes for various operations. The sizeof operator makes these sizes available to your program as a constant. So for example, if you do sizeof(int) the compiler knows how big an int is (in bytes) and will insert that value instead. The same applies for more complex things like sizeof(myVariable) with myVariable being of type MyClass: the compiler does know how much space MyClass takes up and thus can insert that value.

The point is that this evaluation takes places at compile-time: the result is a number. During runtime, the evaluation does not need to be done again.

DarkDust
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It means int j=sizeof(int); would be compiled to int j=4;

I have read the compiled assembly, there is no actually calc during execution!

Chakra
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jerry
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    This is wrong, at least partially. `sizeof(int)` is not always 4, it is platform-dependent; moreover, the result should not be assigned to an `int`, but to a `size_t` variable. – Jean-Philippe Pellet Apr 30 '11 at 09:48