What's the difference between push_back({ "George", 1 }) and push_back(Student("Jack", 10));

Question

I was just wondering if there's a difference between the following two lines:

objStudents.push_back({ "George", 1 });
objStudents.push_back(Student("Jack", 10));

Is one way more efficient than the other? Or is it just a different syntax that completes the same task?

If you want to know if one is more efficient, compile both ways with optimization enabled and test them. — John Zwinck, Oct 25 '20 at 11:27
The difference is the first one pushes "George", the other "Jack" :) What would really help to answer the question is a [mcve] - a complete, short program that we can compile and run. — rustyx, Oct 25 '20 at 11:41
Though the code isn't identical, there are only minor differences: [Test on Compiler Explorer](https://godbolt.org/z/3d974r). I would consider this as less to be concerned about performance... — Scheff's Cat, Oct 25 '20 at 11:50
@GeorgeAustinBradley: `emplaceXXX()` methods don't take a constructed object, but rather - arguments for construction, and construct the object in-place within the container. — einpoklum, Oct 26 '20 at 14:08

Eugene · Accepted Answer · 2020-10-26T14:04:40.903

Assumming Student has a constructor with the right arguments, the difference is that while objStudents.push_back(Student("Jack", 10)); always compiles, objStudents.push_back({ "George", 1 }); only compiles if the constructor is not declared explicit. On the other hand, if Student is an aggregate (e.g. struct Student { string s; int i; };), then only objStudents.push_back({ "George", 1 }); compiles.

Both forms are equally efficient if they compile - both construct Student from e.g. "George", 1 followed by invoking move (if exists) or copy constructor.

In the case with a constructor, a more efficient form, which compiles even if the constructor is explicit, is objStudents.emplace_back("George", 1);, which constructs Student directly inside the vector and avoids the extra move or copy. Of course, "more efficient" here is theoretical - a compiler may be able to optimize away the extra operation in the form with push_back.

I realized I missed the case of aggregates - added it now. – Eugene Oct 26 '20 at 13:50 — Eugene, Oct 26 '20 at 13:50

What's the difference between push_back({ "George", 1 }) and push_back(Student("Jack", 10));

1 Answers1