如何在C ++中声明一个指向常量的指针? [英] How to declare a pointer to pointer to constant in C++?

问题描述

我正在尝试编写一个函数来解析命令行参数.这是函数声明:

I'm trying to write a function to parse command line arguments. This is the function declaration:

void parse(int, char const **);

以防万一，我还使用typedef const char cchar尝试了(const char)**，const char **和cchar **.但是，如果我将char **传递给函数，所有这些(如所期望的，因为它们应该都相同)会导致错误，如:

Just in case, I have also tried (const char)**, const char **, and cchar ** using a typedef const char cchar. However, all of these (as expected, since they should all be identical) result in an error if I pass a char ** into the function, as in:

void main(int argc, char **argv) {
    parse(argc, argv);

我从GNU编译器中得到的错误是error: invalid conversion from 'char**' to 'const char**'，而从Clang中得到的错误是candidate function not viable: no known conversion from 'char **' to 'const char **' for 2nd argument.

The error I get from GNU's compiler is error: invalid conversion from 'char**' to 'const char**' and the one from Clang is candidate function not viable: no known conversion from 'char **' to 'const char **' for 2nd argument.

我已经看到这样的解决方案，建议将其声明为指向char的const指针(const char * const *)，但是我不希望任何一个指针都为const，因为我希望能够修改该指针，以便进行迭代在使用for(; **argv; ++*argv)的参数上.我该如何声明指向const char的非const指针的非const指针"?

I have seen such solutions suggested as declaring a pointer to a const pointer to char (const char * const *), but I don't want either pointer to be const because I want to be able to modify the pointer so I can iterate over an argument using for(; **argv; ++*argv). How can I declare a "non-const pointer to non-const pointer to const char"?

推荐答案

最安全的签名可以防止修改参数，同时允许任何其他const组合调用该函数:

The safest signature that prevents modification of the arguments whilst allowing any other const combination to call the function is this:

parse(int argc, char const* const* argv);

这意味着argv是const的指针 是const char

您可以愉快地遍历这样的参数:

You can happily iterate over the parameters like this:

for(auto arg = argv + 1; *arg; ++arg)
{
    if(!std::strcmp(*arg, "--help"))
        return print_help();
    else if(!std::strcmp(*arg, "-v") || !std::strcmp(*arg, "--verbose"))
        verbose_flag = true;
    // ... etc...
}

请注意，由于字符数组的数组以 null终止，因此不需要接受变量int argc.

Notice there is no need to accept the variable int argc because the array of character arrays is null terminated.

所以我通常使用这个:

struct config
{
    // program options and switches
};

config parse_commandline(char const* const* argv);

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