默认lambda参数中的访问模板类参数

Question

I'm writing a simple universal pool. The template class takes a parameter in the constructor that is a factory function to instantiate objects in the pool as needed.

template<typename T>
struct Foo {
    std::function <T*()> factory_;
    Foo(std::function<T*()> factory): factory_(factory) {}
};

This compiles fine, but I wanted to make a default argument for the constructor using lambda:

Foo(std::function<T*()> factory = [](){return new T();} ): factory_(factory) {}

This does not compile - it says that T is unknown. Is there a way to make lambda expressions aware of class template parameters? I tried using typedef , but to no avail.

Answer 1

This should work as proposed, however you might have hit a bug in VC++ (it's not exactly C++11 ready yet).

As a workaround, you might try to replace your lambda by a private static method instead.

template <typename T>
class LIFOPool
{
    std::function <T*()> factory_;
    //...details irrelevant

    static T* DefaultMake() { return new T{}; }
public:
    LIFOPool(std::function<T*()> factory = DefaultMake) : factory_(factory) {}
    //...details irrelevant
};