C++ copy and move constructors with lambda expressions

Question

I've bumped into something strange with C++ copy and move constructors, here when passing to the lambda expression both the copy and move constructors get executed. Strangely though, when I change the declared type of the lambda to auto or use the regular_test function, I get the expected behaviour (copy construction only). Does anyone understand why this is? (tested with both clang and gcc, not msvc)

#include <iostream>
#include <iomanip>
#include <functional>

using namespace std;

struct Test {
    inline Test() {
        cout << setw(20) << "constructor ";
        PrintAddress();
    }

    Test(const Test&) {
        cout << setw(20) << "copy constructor ";
        PrintAddress();
    }

    Test& operator=(const Test&) {
        cout << setw(20) << "copy assignment ";
        PrintAddress();
        return *this;
    }

    Test(Test&& other) {
        cout << setw(20) << "move constructor ";
        PrintAddress();
    }

    Test& operator=(Test&&) {
        cout << setw(20) << "move assignment ";
        PrintAddress();
        return *this;
    }

    virtual ~Test() {
        cout << setw(20) << "destructor ";
        PrintAddress();
    }

    void PrintAddress() {
        cout << "Test&: " << this << endl;
    }
};

Test regular_test (Test t) {
    cout << "regular_test" << endl;
    return t;
}

int main() {
    cout << "start" << endl;
    function<Test(Test)> lambda_test = [] (Test t) {
        cout << "lambda_test" << endl;
        return t;
    };
    Test t;
    lambda_test(t);
    //regular_test(t);
    cout << "done" << endl;
    return 0;
}

start
        constructor Test&: 0x7fffef6faf28
   copy constructor Test&: 0x7fffef6faf08
   move constructor Test&: 0x7fffef6fade8
lambda_test
   move constructor Test&: 0x7fffef6faf10
         destructor Test&: 0x7fffef6fade8
         destructor Test&: 0x7fffef6faf10
         destructor Test&: 0x7fffef6faf08
done
         destructor Test&: 0x7fffef6faf28

auto: function<Test(Test&)>

CGi03
– CGi03

2022-09-03 13:10:46 +00:00
Commented Sep 3, 2022 at 13:10 — CGi03
– CGi03, Commented Sep 3, 2022 at 13:10

apple apple · Accepted Answer · 2022-09-03 13:08:09Z

1

because std::function::operator() is defined based on the template type of class.

R operator()( Args... args ) const; // Args are parameter of the *class*

so std::function<Test(Test)> would have Test operator()(Test), which would make a copy by itself, then forward to lambda_test (a move)

answered Sep 3, 2022 at 13:08

apple apple

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2 Comments

David Carpenter Over a year ago

ok but why does changing function<Test(Test)> lambda_test to auto lambda_test fix the problem?

apple apple Over a year ago

@DavidCarpenter because auto doesn't add another wrapper. (the lambda is not even copied)

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