C++17 in Detail: A Deep Dive/

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Reference Lifetime Extension

Reference Lifetime Extension

This lesson deals with the lifetime of temporary objects.

We'll cover the following...

What happens in the following case:

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#include <iostream>
#include <vector>
using namespace std;
std::vector<int> GenerateVec() {
  return std::vector<int>(5, 1); 
}
int main() {
  const std::vector<int>& refv = GenerateVec();
  cout << refv.size();
}

Is the above code safe?

Yes - the C++ rules say that the lifetime of a temporary object bound to a const reference is prolonged to the lifetime of the reference itself.

Here’s a full example quoted from the standard (Draft C++17 - N4687) 15.2 Temporary objects [class.temporary]:

Example:

struct S {
 S();
 S(int);
 friend S operator+(const S&, const S&);
 ~S();
};
S obj1;
const S& cr = S(16)+S(23);
S obj2;

The expression S(16) + S(23) creates three temporaries: a first temporary ...