C++14: Generic lambda with generic std::function as class member

Question

Consider this pseudo-snippet:

class SomeClass
{
public:
    SomeClass()
    {
        if(true)
        {
            fooCall = [](auto a){ cout << a.sa         


        

Answer 1

I was not able to store std::function<> as a generic lambda in the class directly as a member. What I was able to do was to specifically use one within the class's constructor. I'm not 100% sure if this is what the OP was trying to achieve but this is what I was able to compile, build & run with what I'm suspecting the OP was aiming for by the code they provided.

template
class test {
public: // While testing I changed this to public access...
        // Could not get object below to compile, build & run
    /*template
    static std::function fooCall;*/
public:
   test();
};

template
test::test() {
    // This would not compile, build & run
    // fooCall = []( T t ) { std::cout << t.sayHello(); };

    // Removed the variable within the class as a member and moved it here
    // to local scope of the class's constructor
    std::function fooCall = []( auto a ) { std::cout << a.sayHello(); };
    T t; // created an instance of 
    fooCall(t); // passed t into fooCall's constructor to invoke the call.
}

struct A {
    std::string sayHello() { return "A say's Hello!\n"; }
};

struct B {
    std::string sayHello() { return "B say's Hello!\n"; }
};


int main() {
    // could not instantiate an object of SomeClass with a member of
    // a std::function<> type that is stored by a type of a generic lambda.

    /*SomeClass someA;
    SomeClass someB;
    someA.foo();
    someB.foo();*/

    // Simply just used the object's constructors to invoke the locally stored lambda within the class's constructor.
    test a;
    test b;

    std::cout << "\nPress any key & enter to quit." << std::endl;
    char c;
    std::cin >> c;

    return 0;
}

With the appropriate headers the above as is should compile, build & run giving the output below (At least in MSVS 2017 on Windows 7 64bit did); I left comments where I ran into errors and tried multiple different techniques to achieve a working example, errors occurred as others suggested and I found even more while working with the above code. What I was able to compile, build and run came down to this simple bit of code here without the comments. I also added another simple class to show it will work with any type:

template
class test {
public:
    test();
};

template
test::test() {
    std::function fooCall = []( auto a ) { std::cout << a.sayHello(); };
    T t;
    fooCall( t );
}

struct A {
    std::string sayHello() { return "A say's Hello!\n"; }
};

struct B {
    std::string sayHello() { return "B say's Hello!\n"; }
};

struct C {    
    int sayHello() { return 100; }
};

int main() {
    test testA;
    test testB;
    test testC;

    std::cout << "\nPress any key & enter to quit." << std::endl;
    char c;
    std::cin >> c;

    return 0;
}

Output:

A say's Hello!
B say's Hello!
100

Press any key & enter to quit

I don't know if this will help the OP directly or indirectly or not but if it does or even if it doesn't it is still something that they may come back to and build off of.