在C ++中会产生与typename和typedef相关的错误的模板类？

Question

I have a class Tran that contains an instance of a class Car. If the program includes typename before typedef , it produces an error: "expected nested-name-specifier before typedef ". If it does not include typename , it produces an error: "need 'typename' before 'Tran::CarType::Model' because 'Tran::CarType' is a dependent scope". What is causing this problem?

#include "Car.hpp"

template<typename A, typename B, typename C>
class Tran {

    public: 
    typedef Car<A, B> CarType;  //compilation error
    typedef CarType::Model M;   //compilation error

private: 
CarType myCar;
}

Answer 1

The issue is with this line:

typedef CarType::Model M;

Here, note that CarType is defined as

typedef Car<A, B> CarType;

Notice that CarType depends on what A and B are. In fact, it's called a dependent type because of this.

In C++, if you want to access a type nested inside of a dependent type, you have to explicitly tell the compiler that you are looking inside of a dependent type for the name of some other type. Therefore, this line is an error:

typedef CarType::Model M;

Because there is no indication that Model is a type name. To fix this, change the line to read

typedef typename CarType::Model M;

Here, the typename keyword indicates to C++ that Model is the name of a type nested inside the dependent type CarType .

Hope this helps!

Answer 2

The correct order is typedef typename and not the other way around. You only need it in the second declaration, as it is a dependent name and the compiler cannot know during the first phase of lookup what CarType::Model is.