If I have a class
class foo {
public:
foo() { // spend some time and do something. }
private:
// some data here
}
Now I have a vector of foo, I want to put this vector into another vector
vector<foo> input; // assume it has 5 elements
vector<foo> output;
Is there ANY performance difference with these two lines?
output.push_back(input[0])
output.emplace_back(input[0])
Is there ANY performance difference with these two lines?
No, both will initialise the new element using the copy constructor.
emplace_back can potentially give a benefit when constructing with more (or less) than one argument:
output.push_back(foo{bar, wibble}); // Constructs and moves a temporary
output.emplace_back(bar, wibble); // Initialises directly
The true benefit of emplace is not so much in performance, but in allowing non-copyable (and in some cases non-movable) elements to be created in the container.
