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luasitdown

forwarding declaration

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// A.h file
class A
{
B * b;
b->fun();
public:
//stuff
}

// B.h
class B
{
A * a;
public:
a->fun();
//stuff
}

how to solve the problem. forwarding declaration not work.

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A* a;
a->fun();


The above requires the definition of A to be available. What is usually done is only keep the A* a; part in the header file, and move the second line to a source file. This way, you can use a forward declaration to make the header self-sustaining, and include all definitions in the source file to make it compile.

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The compiler does not need the full class declaration to make a pointer to it.

Therefore, you can solve your cyclical dependency like so:


// In ClassA.h:

// Use a forward declaration to tell the compiler B is a type
class B;

class A {

B* b;
};

// In ClassA.cpp:

#include "ClassA.h"
#include "ClassB.h"

// somewhere later on...
b->fun();

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