Is it possible to std::move local stack variables?

Question

Please consider the following code:

struct MyStruct
{
    int iInteger;
    string strString;
};

void MyFunc(vector& vecStructs)
{
    MyStr         


        

Answer 1

Your example can be reduced to:

vector vec;
string str; // populate with a really long string
vec.push_back(std::move(str));

This still raises the question, "Is it possible to move local stack variables." It just removes some extraneous code to make it easier to understand.

The answer is yes. Code like the above can benefit from std::move because std::string--at least if the content is large enough--stores it actual data on the heap, even if the variable is on the stack.

If you do not use std::move(), you can expect code like the above to copy the content of str, which could be arbitrarily large. If you do use std::move(), only the direct members of the string will be copied (move does not need to "zero out" the old locations), and the data will be used without modification or copying.

It's basically the difference between this:

char* str; // populate with a really long string
char* other = new char[strlen(str)+1];
strcpy(other, str);

vs

char* str; // populate with a really long string
char* other = str;

In both cases, the variables are on the stack. But the data is not.

If you have a case where truly all the data is on the stack, such as a std::string with the "small string optimization" in effect, or a struct containing integers, then std::move() will buy you nothing.