What causes compiling error with templated class?

Question

I have simple codes for templated and non-templated versions of the same class. Each class has the same data structure (Point). The compiling problem happens for the templated class when defining the function with the structure Point externally. I compiled the codes with Visual Studio.

//non-templated class is defined without a compiling error.
class Geometry1 {
public:
    struct Point
    {
        double x = 1, y = 2, z = 3;
    };
public:
    Point GetAPoint()//Function is defined internally
    {
        Point test;
        return test;
    }

    Point GetAnotherPoint();//Function will be defined externally
};

Geometry1::Point  Geometry1::GetAnotherPoint()//This externally defined function is fine with Point
{
    Point test;
    return test;
}


//Now a templated version of Geometry1 is defined. Compiling error happens!!!
template<typename type>
class Geometry2{
public:   
    struct Point
    {
        type x=1, y=2, z=3;   
    };
public:
    Point GetAPoint()//No compiling error
    {
        Point test;
        return test;
    }

    Point GetAnotherPoint();//Function will be defined externally WITH compiling error 
};

template<class type>
Geometry2<type>::Point  Geometry2<type>::GetAnotherPoint()//This externally defined function causes compiling error with Point: Error   C2061   syntax error: identifier 'Point'
{
    Point test;
    return test;
}

int main()
{
    Geometry1 geo1;//There is compiling error for this class
    Geometry1::Point p1 = geo1.GetAnotherPoint();
    
    Geometry2<double> geo2;//Compiling error happens for this class
    Geometry2<double>::Point p2 = geo2.GetAPoint();
}

Complete error messages are:

Severity    Code    Description Project File    Line    Suppression State
Error   C2061   syntax error: identifier 'Point'    TestClasstypedef    TestClasstypedef.cpp    51  
Error   C2143   syntax error: missing ';' before '{'    TestClasstypedef    TestClasstypedef.cpp    52  
Error   C2447   '{': missing function header (old-style formal list?)   TestClasstypedef    TestClasstypedef.cpp    52  
Error   C2065   'geo1': undeclared identifier   TestClasstypedef    TestClasstypedef.cpp    60  
Error   C2065   'geo2': undeclared identifier   TestClasstypedef    TestClasstypedef.cpp    63

I found that if Geometry2::GetAnotherPoint() is defined internally like Geometry2::GetAPoint(), the problem will be completely solved.

Could anyone help me to figure out my mistake? Thank you in advance!

Thank you very much for the quick response! It is completely solved. Best wishes!!!! — Trung, Feb 15 '22 at 08:28

Jason · Accepted Answer · 2022-02-15T08:47:21.337

The problem is that Geometry2<type>::Point is a dependent qualified name. Moreover, this dependent name is a type so you need to tell this to the compiler which you can do by adding the keyword typename before Geometry2<type>::Point. So the modified code looks like:

template<class type>
typename Geometry2<type>::Point Geometry2<type>::GetAnotherPoint()//note the keyword typename here
{
    Point test;
    return test;
}

What causes compiling error with templated class?

1 Answers1