函数参数中的C ++类类型

Question

Say I have two classes setup as shown below. Basically I want to pass the class1 object defined in main() to a method in the second class. When I try to compile this, it says that class1 has not been declared. Can anyone explain why this doesn't work and how I can fix it?

//class1.h:

class class1
{
public:
    class1();
    void method1();
private:
    int myNumber;
};

//class1.cpp has the implementation for method1();

//class2.h:

class class2
{
public:
    class2();
    void method2(class1 myclass);
};

//class2.cpp:

#include "class1.h"
#include "class2.h"

int main( void )
{
    class1 myclass;
    class2 anotherClass;

    anotherClass.method2(myclass);

    return 0;
}

void class2::method2(class1 myclass)
{
    return;
}

Answer 1

You are missing the class keyword in your class definitions.

instead of class1{...} you need class class1{...} , etc.

Answer 2

In class2.h , the compiler does not "know" how class1 is defined. There are two possible solutions:

• Add an #include "class1.h" in class2.h
• Add a so-called forward declaration of class1 in class2.h by simply declaring class class1; at the top of the header. You then need to change your method2 to either expect a reference or a pointer to class1 . If you then add #include "class1.h" in class2.cpp , everything will work.

Answer 3

the only clean solution is to include the header defining class class1 before the definition of class class2 .

Note that a forward declaration (al la Gnosophilon's answer) won't help here, as it only declares the class, but does not define it. This means that you can use a pointer or reference to the class, but not more, in particular you cannot pass class1 myclass by value as you try to do.