No matching member function error inside lambda expression?

Question

This is quite a strange error to me. Check the code below:

void test(void){
    vector<string> v;
    v.push_back("hello");
    auto fn=[=](){
        v.push_back("world");
    };
}

The first push_back method passed the compilation but the second failed, yielding the error:

Error:no matching member function for call to 'push_back'

The compiler note is:

**Note:(687, 36) candidate function not viable: 'this' argument has type 'const vector' (aka 'const vector, allocator > >')

But the method is not marked const**.
Well I am not using any const argument and I cannot figure what the compiler is trying to tell me. Could anybody help me?

Answer 1

Lambda call operator member functions are const by default. If you want a mutable call operator, say mutable:

auto fn = [=]() mutable {
//              ^^^^^^^
    v.push_back("world");
};

Having const be the default forces you to be explicit about the fact that you mean to capture a copy of the vector and mutate that copy, rather than the original vector v.

By contrast, variables that are captured by reference can be mutated by const-qualified member functions:

auto fn = [&]() {
//        ^^^
    v.push_back("world");  // modifies original "V"!
};

(This is essentially because const T is the same as T when T = U &; there are no "constant references" in C++.)

Answer 2

capture by value is const use the keyword mutable (doesn't change the original vector):

auto fn = [=]() mutable {
    v.push_back("world");
};

or by reference (changes the original vector):

auto fn = [&]() {
    v.push_back("world");
};