How do I capture some (but not all) member variables of a class with C++ lambda

Question

The following dummy example may not really make sense in real world. But it explains the question. I have a class Foo with members firstname and lastname. The function ForEachMessage takes a lambda. I want it to capture only the firstname of Foo but not lastname. How do I achieve that?

#include <iostream>
#include <vector>
#include <functional>
using namespace std;

vector<string> messagesList;
void ForEachMessage(function<void(const string&)>callBack)
{
    for (const auto& str : messagesList) {
        callBack(str);
    }
}

class Foo {
public:
    std::string firstname;
    std::string lastname;
    void say() {
        ForEachMessage([this](const std::string& someMessage)
        {
            cout << firstname << ": " <<  someMessage << endl;
        });

        // Error: firstname in capture list doesn't name a variable
        // ForEachMessage([firstname](const std::string& someMessage)
        // {
        //    cout << firstname << ": " <<  someMessage << endl;
        // });

        // Error: expect ',' or ']' in lambda capture list
        // ForEachMessage([this->firstname](const std::string& someMessage)
        // {
        //    cout << firstname << ": " <<  someMessage << endl;
        // });
    }
};

int main(int argc, const char * argv[]) {
    Foo foo;
    foo.firstname = "Yuchen";
    foo.lastname = "Zhong";
    messagesList.push_back("Hello, World!");
    messagesList.push_back("Byebye, World!");
    foo.say();
    return 0;
}

"Doesn't work" is the least useful problem description in the world. — molbdnilo, Mar 01 '16 at 17:32
@molbdnilo, thanks for the comments. I added the error message as well. — Yuchen, Mar 01 '16 at 17:35
[Quick fix](http://coliru.stacked-crooked.com/a/131b92770d94e458). Wonder if there's a better way though. — Baum mit Augen, Mar 01 '16 at 17:36
For what it's worth, cherry-picking variables is probably not worth the effort. The compiler can figure things out pretty well and shouldn't capture variables not used by the lambda. — Colin Basnett, Mar 01 '16 at 17:37

score 5 · Accepted Answer · edited May 23 '17 at 12:31

5

You can use the C++14 named capture for this:

ForEachMessage([bla=firstname](const std::string& someMessage)
    {
        cout << bla << ": " <<  someMessage << endl;
    });

(See it live) To avoid a copy, you may also capture by reference with [&bla=firstname] instead. For a discussion on how to capture by const&, see this question and in particular this answer.

(Remark: The local name can, but need not be different from the member name.)

edited May 23 '17 at 12:31

Community

1
1

answered Mar 01 '16 at 17:41

Baum mit Augen

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Looks like you could even use `[&firstname=firstname]` – Simon Kraemer Mar 01 '16 at 17:46
@SimonKraemer Indeed, you beat my edit by seconds. Thx for the input. – Baum mit Augen Mar 01 '16 at 17:47
1

@BaummitAugen I just tested if it's possible to make the reference `const` to avoid it being altered within the lambda. `[&firstname=static_cast(firstname)]` does the job. – Simon Kraemer Mar 01 '16 at 17:51

Nard · Answer 2 · 2016-03-01T18:52:55.573

3

Create a reference to firstname

const std::string& rfn = firstname;

And capture the reference

ForEachMessage([rfn](const std::string& someMessage)
{
  cout << rfn << ": " << someMessage << endl;
}

edited Mar 01 '16 at 18:52

answered Mar 01 '16 at 17:38

Nard

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Oops. Thanks for fixing mine. M( – Baum mit Augen Mar 01 '16 at 20:32

How do I capture some (but not all) member variables of a class with C++ lambda

2 Answers2