What is this way that something weird return?

woohyeon (60)
I was solving Leetcode, and I saw someone's code.
and I saw an interesting code that when a function returns vector<int>, It returns like this way.

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vector<int> retVec()
{
	return { 1, 3, 5 };
}


and It's really working.
I think it's more efficient and simple and faster than a normal way.
you guys think It's really do? or Not?


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// a normal way
vector<int> retVec()
{
	vector<int> tmp;
	tmp.push_back(1);
	tmp.push_back(3);
	tmp.push_back(5);
	return tmp;
}
Last edited on
coder777 (8444)
It calls the constructor which take an initializer list. See:

http://www.cplusplus.com/reference/vector/vector/vector/

It's C++11 the constructor (6).
keskiverto (10402)
In other words it is closer to:
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// a normal way
vector<int> retVec()
{
	vector<int> tmp { 1, 3, 5 };
	return tmp;
}

If you start empty and do push_back, then you most likely invoke reallocations within the vector (unless the compiler miraculously is able to optimize them away). You could avoid those with:
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// a normal way
vector<int> retVec()
{
	vector<int> tmp;
	tmp.reserve( 3 );
	tmp.push_back(1);
	tmp.push_back(3);
	tmp.push_back(5);
	return tmp;
}

// or
vector<int> retVec()
{
	vector<int> tmp( 3 );
	tmp[0] = 1;
	tmp[1] = 3;
	tmp[2] = 5;
	return tmp;
}
Last edited on
dhayden (5798)
return { 1, 3, 5 };

where are you going to get those values. What you most definitely do NOT want to do is:
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vector<int> retVec()
{
	vector<int> tmp;
	tmp.push_back(1);
	tmp.push_back(3);
	tmp.push_back(5);
	return {tmp[0], tmp[1], tmp[2] };  // Look how smart I am!
}


This is actually slower than
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// a normal way
vector<int> retVec()
{
	vector<int> tmp;
	tmp.push_back(1);
	tmp.push_back(3);
	tmp.push_back(5);
	return tmp;
}

This is because of "copy elision" (did I spell that right?). The compiler will recognize that tmp is the return vector and will store it in the return value. So when you return, it doesn't have to make a copy at all.

But this is like optimizing a ventilation system by examining how much air is moved by flies. It's a tiny gain and probably not worth any consideration.
woohyeon (60)
Thank you, guys!!
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