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Move semantics can be useful when the compiler cannot use RVO and NRVO. But in which case can't the compiler use these features?
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Compilers often perform Named Return Value Optimization (NRVO) in such cases, but it is not guaranteed.
There is no way for RVO optimization to break the promise of a const , so there's no problem: RVO can be performed. However, move semantics is affected by the const . It effectively disables move semantics, that is, calls of a T(T&&) constructor or move assignment operator.
In the context of the C++ programming language, return value optimization (RVO) is a compiler optimization that involves eliminating the temporary object created to hold a function's return value. RVO is allowed to change the observable behaviour of the resulting program by the C++ standard.
The neat thing about RVO is that it makes returning objects free. It works via allocating memory for the to-be-returned object in the caller's stack frame. The returning function then uses that memory as if it was in its own frame without the programmer knowing / caring.
The answer is that it is compiler and situation dependent. E.g. control flow branching might confuse optimizers. Wikipedia give this example:
#include <string>
std::string f(bool cond = false) {
std::string first("first");
std::string second("second");
// the function may return one of two named objects
// depending on its argument. RVO might not be applied
return cond ? first : second;
}
int main() {
std::string result = f();
}
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