对C ++指针和参考主题的困惑

Question

What is the difference between the following parameter passing mechanisms in C++?

void foo(int &x) 
void foo(int *x)
void foo(int **x)
void foo(int *&x)

I'd like to know in which case the parameter is being passed by value or passed by reference.

Answer 1

void foo(int &x)

passes a reference to an integer. This is an input/output parameter and can be used like a regular integer in the function. Value gets passed back to the caller.

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void food(int *x)

passes a pointer to an integer. This is an input/output parameter but it's used like a pointer and has to be dereferenced (eg *x = 100; ). You also need to check that it's not null.

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void foo(int **x)

passes a pointer to a pointer to an integer. This is an input/output parameter of type integer pointer. Use this if you want to change the value of an integer point (eg *x = &m_myInt; ).

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void foo(int *&x)

passes a reference to a pointer to an integer. Like above but no need to dereference the pointer variable (eg x = &m_myInt; ).

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Hope that makes sense. I would recommend using typedefs to simplify the use of pointers and reference symbols.

Answer 2

Just adding: I think your spacing is misleading. Maybe things get a bit clearer if you change it.

The , &, * and so on is part of the type, so keep it with the type:

void foo(int& x) 
void foo(int* x)
void foo(int** x)
void foo(int*& x)

int& is an reference to an int, int* is a pointer to an int, int** is a pointer to an pointer to an int, and so on. You still need to read the types from right to left - int*& being a reference to an pointer to an int. But that's consistent.

I think this is easier to read and represents better what is meant.