CODEWITHSUNDEEP
c++functiontemplateslambdaparameters
DoingItSideways
DoingItSideways

Reputation: 13

Lambda-function as a parameter

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.

Upvotes: 1

Views: 157

Answers (2)

molbdnilo
molbdnilo

Reputation: 66459

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);
    }
}

Upvotes: 5

spectras
spectras

Reputation: 13552

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.

Upvotes: 1

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