C++ lambda copy value in capture-list

Question

I have a program as below:

int main()
{
    int val = 4;
    auto add = [val](int a)->int{
        val += 2;
        return a+val;
    };
    cout << add(3) << endl;
    cout << val << endl;
    return 0;
}

There's a compiling error in Xcode: Cannot assign to a variable captured by copy in a non-mutable lambda.

My question is: if we choose to use the copy (using "=" or value name), can't this value be assigned a new value or changed?

possible duplicate of [Why does C++0x's lambda require "mutable" keyword for capture-by-value, by default?](http://stackoverflow.com/questions/5501959/why-does-c0xs-lambda-require-mutable-keyword-for-capture-by-value-by-defau) — AndyG, Nov 04 '14 at 21:17

Columbo · Accepted Answer · 2014-11-04T21:08:48.323

Inside a lambda, captured variables are immutable by default. That doesn't depend on the captured variables or the way they were captured in any way. Rather, the function call operator of the closure type is declared const:

This function call operator or operator template is declared const (9.3.1) if and only if the lambda-expression’s parameter-declaration-clause is not followed by mutable.

Therefore, if you want to make the captured variables modifiable inside the body, just change the lambda to

auto add = [val] (int a) mutable -> int {
    val += 2;
    return a+val;
};

so the const-specifier is removed.

T.C. · Answer 2 · 2014-09-13T21:23:14.950

12

The operator () of a lambda is implicitly const unless the lambda is declared mutable - and you can't modify the data members in a const member function. This happens regardless of the type of the capture.

edited Sep 13 '14 at 21:23

answered Sep 13 '14 at 21:16

T.C.

133,968
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421

score 4 · Answer 3 · edited Apr 23 '18 at 01:23

4

Just capture it by reference, it will work !!

 auto add = [&val](int a) -> int{ 
       //
}

edited Apr 23 '18 at 01:23

Roushan

4,074
3
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answered Apr 22 '18 at 20:18

5

Please, nobody do this. It will work until it explodes. – Steazy Sep 12 '19 at 22:57
3

@Steazy why might it explode? – NateW Apr 08 '21 at 19:40
2

If the object that `val` refers to goes out of scope, this will "explode". For example, this could be the case when you return the lambda from a function. – David Scherfgen Nov 07 '21 at 19:24
1

To echo other commenters. Don't do this. Effective Modern C++ by Scott Meyers, Item 31 has a paragraph specifically saying *not* to do this – xdavidliu Jun 01 '23 at 13:44

C++ lambda copy value in capture-list

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