Template type deduction in C++ for Class vs Function?

Question

Why is that automatic type deduction is possible only for functions and not for Classes?

Answer 1

In case of a function call, the compiler deduces the template type from the argument type. For example the std::max-function. The compiler uses the type of the arguments to deduce the template parameter. This does not allways work, as not all calls are unambigous.

int a = 5;
float b = 10;

double result1 = std::min( a, b ); // error: template parameter ambigous
double result2 = std::min< double >( a, b ); // explicit parameter enforces use of conversion

In case of a template class, that may not allways be possible. Take for instance this class:

template< class T>
class Foo {
public:
    Foo();
    void Bar( int a );
private:
    T m_Member;
};

The type T never appears in any function call, so the compiler has no hint at all, what type should be used.