c++c++11g++4.8

Tuple as a return type, are unaccessed values optimized out?

I'm returning three items from a function in the form of a std::tuple. 

... myFunction()
{
    ...
    return std::tuple< int, unsigned long long, unsigned int >{ errorCode, timeStamp, sizeOfBuffer };
}

Since return values have to be accessed using std::get or std::tie, does the compiler optimize for unused values (g++ 4.8)?

Solution

  • Yes, it can

    #include "stdio.h"
#include <tuple>

std::tuple<int, unsigned long long, unsigned int> myFunction()
{
    return std::tuple<int, unsigned long long, unsigned int>{ 1, 2, 3 };
}

int f()
{
  return std::get<0>(myFunction());
}

    Becomes

    myFunction():
    movq    %rdi, %rax
    movl    $3, (%rdi)
    movq    $2, 8(%rdi)
    movl    $1, 16(%rdi)
    ret
f():
    movl    $1, %eax
    ret