C ++前向声明

Question

#include <iostream>
#include <cstring>
using namespace std;

class Obj;


class Test {
    friend class Obj;
public:
    Test()
    {

    }
    ~Test()
    {

    }
    void foo()
    {
        //print();
            //Obj::print();
            //Obj x;
            //x.print();
    }
};

class Obj {
public:
    void print()
    {
        cout << "print here" << endl;
    }
};

int main()
{
    Test test;
    test.foo();
    return 0;
}

Quick question,how can I call print the correct way in Test::foo() ?

Answer 1

You need to define the member function after the definition of Obj :

class Test { 
public:
    void foo();
};

class Obj {
public:
    void print() { }
};

void Test::foo() { 
    Obj o;
    o.print();
}

Answer 2

As mentioned by james you should define the member function after the definition of Obj. Also you are calling Obj::print, but print is not a static member function so you must call it on an instance of Obj not Obj itself.

If you really do want print to be a static member, declare it so.

class Obj {
public:
    static void print(){ blah }
}

Also you do not need to make Obj a friend in order to access its public methods.

Also can OP please define "correct way", I was assuming you wanted it to be a static member function, james' answer is correct if you want one instance of Obj per instance of Test.

UPDATED OP, as per your comment you must have the declaration of Obj along with print BEFORE using it within Test. This can be achieved in many ways:

• move the entire class Obj defintion (and declaration) before Test
• declare Obj's methods with its class definition and define them later.
• declare Test like you have and Define Test as per James' post (after Obj).

The following works fine:

#include <iostream>
#include <cstring>
using namespace std;

class Obj {
public:
    static void print()
    {
        cout << "print here" << endl;
    }
};

class Test {
public:
    Test()
    {

    }
    ~Test()
    {

    }
    void foo()
    {
            Obj::print();
    }
};

int main()
{
    Test test;
    test.foo();
    return 0;
}

However it is always nicer (in my opinion) to separate declaration from definition for all but the most trivial of cases.