Is it possible in C++11 / 14 syntax to write an lambda function which will see parent variables?

Question

in abstraction:

int i = 1;
auto go = [] () {
    return i;
};

Is it possible to make something like that in modern C++ syntax?

Answer 1

Formally speaking, the ability to access the surrounding context is the key difference between a function (which cannot) and a closure (which can). Depending on the languages this capture of the environment may occur via copy or reference.

In C++11 (and beyond), lambdas are closures and as usual with C++ we have a fine-grained way of specifying how the capture is done:

• by copy: implicitly [=]() { return i; } or explicitly [i]() { return i; }
• by reference: implicitly [&]() { return i; } or explicitly [&i]() { return i; }

and C++14 even introduces generalized lambda captures, so you can capture:

• by move: existing variable [i = std::move(i)]() { return i; }
• or create a new variable [i = 1]() { return i; }

The square brackets delimit the capture list.

Answer 2

Sure, depends on whether you want to capture it by value:

auto go = [i] () {
    return i;
};

Or by reference:

auto go = [&i] () {
    return i;
};