C/C++:Const Struct中的指针

Question

如何在函数fn中强制obj-> val1指向的内存的常量？

#include <iostream>

struct foo {
    int* val1;
    int* val2;
    int* val3;
};

void fn( const foo* obj )
{
    // I don't want to be able to change the integer that val1 points to
    //obj->val1 = new int[20]; // I can't change the pointer,
    *(obj->val1) = 20; // But I can change the memory it points to...
}

int main(int argc, char* argv[])
{
    // I need to be able to set foo and pass its value in as const into a function
    foo stoben;
    stoben.val1 = new int;
    *(stoben.val1) = 0;
    std::cout << *(stoben.val1) << std::endl; // Output is "0"
    fn( &stoben );
    std::cout << *(stoben.val1) << std::endl; // Output is "20"
    delete stoben.val1;
    return 0;
}

这里的代码非常自我解释.我需要能够创建一个非const对象并用数据填充它,然后将其传递给无法修改此数据的函数.我怎么能这样做？

我知道我可以传入一个const int指针,但理论上,这个类还包含其他几个指针,我也需要在"fn"中使用.

谢谢,

格里夫

Answer 1

由于您标记为C++,您可以创建该成员private并创建一个返回a的访问者const int *.您最初可以通过构造函数或friend函数设置成员.

Answer 2

我不是 C++ 人员，但在 C 中，我会通过两种不同的结构声明来处理这个问题，一种是公共的，一种是私有的：

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#include <stdio.h>\n#include <stdlib.h>\n\nstruct private_foo {\n    int* val1;\n    int* val2;\n    int* val3;\n};\n\nstruct public_foo {\n    int const * const val1;\n    int const * const val2;\n    int const * const val3;\n};\n\n\nvoid fn( struct public_foo * obj )\n{\n    int local;\n    *(obj->val1) = 20; // compile error\n    obj->val1 = &local; // compile error\n}\n\nint main(int argc, char* argv[])\n{\n    // I need to be able to set foo and pass its value in as const into a function\n    struct private_foo stoben;\n    stoben.val1 = malloc(sizeof(int));\n    if (!stoben.val1) { return -1; }\n    *(stoben.val1) = 0;\n    printf("%d", *(stoben.val1) );\n    fn( (struct public_foo *) &stoben );\n    printf("%d", *(stoben.val1) );\n    free(stoben.val1);\n    return 0;\n}\n

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当我尝试使用 GCC 编译上述内容时，出现以下编译器错误，因为我正在尝试修改只读内存：

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temp.c: In function \xe2\x80\x98fn\xe2\x80\x99:\ntemp.c:20: error: assignment of read-only location\ntemp.c:21: error: assignment of read-only member \xe2\x80\x98val1\xe2\x80\x99\n

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