Error when using lambdas

Question

I have a class:

class InputMap
{
    public:
        template<typename Function>
        void setHotkey(sf::Keyboard::Key hotkey, Function onClick)
        {
            map[hotkey] = onClick;
        }

        void triggerHotkey(sf::Keyboard::Key hotkey)
        {
            if(map.find(hotkey) != map.end())
                map[hotkey]();
        }

    protected:
        std::map <sf::Keyboard::Key, std::function<void()>> map;
};

and when calling the setHotkey function like so:

setHotkey(sf::Keyboard::Left, [=](){TestActor->move(sf::Vector2f(-20, 0));});
setHotkey(sf::Keyboard::Right, [=](){TestActor->move(sf::Vector2f(20, 0));});

I get these errors:

../Tyrant/include/Framework/InputMap.hpp|14|error: ‘void TGE::InputMap::setHotkey(sf::Keyboard::Key, Function) [with Function = TGE::State::setHotkey(sf::Keyboard::Key, Function) [with Function = TestState::enter()::__lambda2]::__lambda1]’, declared using local type ‘TGE::State::setHotkey(sf::Keyboard::Key, Function) [with Function = TestState::enter()::__lambda2]::__lambda1’, is used but never defined [-fpermissive]|
../Tyrant/include/Framework/InputMap.hpp|14|error: ‘void TGE::InputMap::setHotkey(sf::Keyboard::Key, Function) [with Function = TGE::State::setHotkey(sf::Keyboard::Key, Function) [with Function = TestState::enter()::__lambda3]::__lambda1]’, declared using local type ‘TGE::State::setHotkey(sf::Keyboard::Key, Function) [with Function = TestState::enter()::__lambda3]::__lambda1’, is used but never defined [-fpermissive]|
||=== Build finished: 2 errors, 1 warnings ===|

Now I'm guessing I could just compile with -fpermissive but I would like to avoid doing so.

EDIT:

Apparently the error was because the .cpp file for InputMap contained

template<typename Function>
void setHotkey(sf::Keyboard::Key hotkey, Function onClick)
{
    map[hotkey] = onClick;
}

and the header was

template<typename Function>
void setHotkey(sf::Keyboard::Key hotkey, Function onClick);

So I guess it doesn't like the declaration and implementation to be in different files, possibly because of the template? Is there a proper way to do this or am I supposed to have it in the header only?

You haven't provided enough code to reproduce the problem. Here is an example using your code which does not give an error: http://coliru.stacked-crooked.com/a/eca4700e759af6a2 — Vaughn Cato, Nov 09 '13 at 01:03
I have tried adding as much as I possibly can, I can't reproduce the error outside of my actual project. http://ideone.com/LPrUdK Is it possible part of the error may be because inputmap, state, game, statemanager are all in a library and teststate is not? — Mike, Nov 09 '13 at 01:46

defube · Accepted Answer · 2013-11-10T09:14:17.377

It is because of the way template instantiation is performed.

The definition has to be fully visible within the translation unit in which it is instantiated.

Putting the implementation in a .cpp made the definition visible only within that translation unit.

Roughly speaking, one .cpp file per translation unit (though I have seen some strange, sick, things...).

So, to fix your problem, you can move the implementation into the header in which it is declared.

Error when using lambdas

1 Answers1