Lambda-function as a parameter

Question

I am working with lambdas for the first time. I am supposed to write a function walk() which takes a lambda function as a parameter.

In the header I declared said function as:

template<class T>    
void walk(T operation) const;

We are supposed to define the function in an .inl, which I have done like this:

template<class T>
void Sea::Grid2D<T>::walk(T operation) const{

    for(auto a : Sea::Grid2D<T>::grid){
        operation(a);
    }
}

My question comes in at this point, since we are given a test class, which calls our walk() function like this.

    grid.walk([&](int const &cell) { sum += cell; });

This call of the walk function results in the following error:

error: cannot convert 'testWalkAndFilter()::<lambda(const int&)>' to 'int'

43 | grid.walk([&](int const &cell) { sum += cell; });

How would I go about converting my lambda function into the int or needed parameter?

While trying to solve this problem. I've also tried to give the walk function a reference, or a const reference parameter, but nothing has worked so far.

Are the T in Sea::Grid2D<T> and the T in T operation supposed to always be the same type? Currently, you're forcing them to be the same. — alter_igel
– alter_igel, Commented Jun 14, 2021 at 15:28

molbdnilo · Accepted Answer · 2021-06-14 18:54:30Z

You have instantiated Sea::Grid2D<int> - that is, T is int - which gives you:

void Sea::Grid2D<int>::walk(int operation) const {
    for(auto a : Sea::Grid2D<int>::grid) {
        operation(a);
    }
}

which fairly obviously has a typing problem - the type of the operation should not be the same as the type of your grid elements.

Since you have a function template inside of a class template, you need two "levels" of templating, e.g.:

template<class T>
class Grid2D {
    ...
    template <class Fn>
    void walk(Fn operation) const;
    ...
};

...

template<class T>
template <class Fn>
void Sea::Grid2D<T>::walk(Fn operation) const {
    for(auto a : grid) {
        operation(a);
    }
}

spectras · Accepted Answer · 2021-06-14 15:26:58Z

1

You are probably using the name T for 2 different arguments (I guess one at the Grid2D class level and one on this function).

template<class T>
void Sea::Grid2D<T>::walk(T operation) const{

    for(auto a : Sea::Grid2D<T>::grid){

Rename this one to something else. Like, U. But it's better to give them name that reflect the intent, if possible. Like, Callable or Operation.

answered Jun 14, 2021 at 15:26

spectras

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