Is there any difference betwen [=] and [&] in lambda functions?

Question

I was studying lambda function in c++11 recently. But I don't know if there is any difference between [=] and [&]. If there is, what the difference is?

And in these two situation, does this in lambda body has any difference?

Answer 1

The difference is how the values are captured

• & captures by reference
• = captures by value

Quick example

int x = 1; auto valueLambda = [=]() { cout << x << endl; }; auto refLambda = [&]() { cout << x << endl; }; x = 13; valueLambda(); refLambda(); 

This code will print

1 13 

The first lambda captures x by value at the point in which valueLambda is defined. Hence it gets the current value of 1. But the refLambda captures a reference to the local so it sees the up to date value

Answer 2

• & means "capture by reference".
• = means "capture by value".

I replied here because I want to point out one thing:

this pointer is always captured by value. In C++11, this means, that if you want to capture a copy of a variable in a class, such as this->a, it will always be captured by reference in practice. Why?

Consider:

[this]() { ++this->a; } 

this is captured by value, but this is a pointer, so a is referenced through this.

If you want a copy of a member variable, in C++11, do something like this:

auto copy = this->a; [copy]() mutable { ++copy; } 

Beware of this caveat, because it is not intuitive until you think of it.