调用函数后的值通过引用传递给 C++ 中的指针？

Question

i cannot understand why the value of *p and *p1 change after cout their value in the code lines below. thank you for your help.

void g(int *&x) {
    int a = 3;
    x = &a;
}
void h(const int *&x) {
    int b = 2;
    x = &b;
}

int main() {
    int *p = new int;
    *p = 5;

    g(p);
    cout << p << " " << *p << endl; // 0095FD48 3 *why the value change in the next line? *
    cout << p << " " << *p << endl; // 0095FD48 1829755776 
    const int*p1 = p;
    h(p1);
    cout << p << " " << *p << endl; // 0095FD48 2 *in here it happen, too*
    cout << p << " " << *p << endl; // 0095FD48 1829755776 
}

Answer 1

g() and h() assign the address of a local variable to the parameter and thereby expose the address to the caller. When these functions return, the local variable is freed, ie, its place on the stack may be re-used.

Therefore, this is undefined behavior. In your example, it is a case of "use after free".

The calls to cout and its operator<< use some portion of the stack and overwrite the "former local variable".

There are several fixes for your sample code, depending on what you want to achieve:

1. If you want your function to modify the value of *p of the main function, then do not introduce a new integer variable in g() , but access the existing one: void g(int *&x) { *x=3; } void g(int *&x) { *x=3; }

2. Is basically the same as (1), but you without references instead of pointers. void g(int &x) { x=3; // x is no local variable but references the callers argument }

3. Allocate a new variable. void g(int *&x) { x = new int; // allocate int value on heap *x = 3; } void g(int *&x) { x = new int; // allocate int value on heap *x = 3; } Because you pass a pointer as reference, the former pointer value will be overwritten, which may lead to a memory leak.

4. Same as (3) but using a return value and thereby reducing the risk of memory leaks: int * g() { int *x = new int; // allocate int value on heap *x = 3; return x; } int * g() { int *x = new int; // allocate int value on heap *x = 3; return x; }

5. To avoid memory leaks, you may use "automatic pointers", see http://www.cplusplus.com/reference/memory/unique_ptr/