Get size of vector elements at compile time (even if it is empty) using the Visual Studio compiler [duplicate]

Question

Given a std::vector of elements of some type, how can I determine the size of the type of those elements at compile time using the Visual Studio compiler. Using sizeof on the first element is not an option as the vector could be empty.

For clang and gcc I can do something like this:

#include <vector>

template<typename T>
size_t size_of_vector_elements(std::vector<T> vector)
{
    return sizeof(T);
}


int square(int num)
{
    std::vector<unsigned> vector;
    return size_of_vector_elements(vector);
}

Using O2 this will be optimized at compile time to just return 4. MSVC however cannot optimize this.Is there a way to change this code to make it possible?

Answer 1

In c++11 or later you can use decltype(vect)::value_type to get type of elements in std::vector

#include <iostream>
#include <vector>

int main()
{
    std::vector<double> vect;
    std::cout << sizeof(decltype(vect)::value_type); // gives you sizeof(double)
    return 0;
}

Answer 2

Since you want MSVC to generate optimized code for the square function that is similar to what GCC generates, you will have to modify your code.

You can do away with the function template size_of_vector_elements and make use of the fact that sizeof does not evaluate its operand.

Then your modified square function simply becomes this:

int square(int num)
{
    std::vector<unsigned> vector;
    return sizeof(vector[0]);
}

The assembly that MSVC produces for this function at /O2 optimization is:

int square(int) PROC                                    ; square, COMDAT
        mov     eax, 4
        ret     0
int square(int) ENDP     

which is similar to what GCC produces at -O2 optimization.

square(int):
        mov     eax, 4
        ret

See godbolt demo.