如何用C ++模板推导const char字符串的编译时间大小? [英] How to deduce the size of a compile time a const char string with C++ templates?

问题描述

我正在尝试使用 clang ++ 扩展

I am trying to use clang++ to expand the template code from this post. Here is what I come up with.

constexpr unsigned int requires_inRange(unsigned int i, unsigned int len) {
    return i >= len ? throw i : i;
}

class StrWrap
{
    unsigned size_;
    char * const begin_;

public:
    template< unsigned N >
    constexpr StrWrap( const char(&arr)[N] ) : begin_(arr), size_(N - 1) {
        static_assert( N >= 1, "not a string literal");
    }

    constexpr char operator[]( unsigned i ) {
        return requires_inRange(i, size_), begin_[i];
    }

    constexpr operator const char *() {
        return begin_;
    }

    constexpr unsigned size() {
        return size_;
    }
};

constexpr unsigned count( StrWrap str, char c, unsigned i = 0, unsigned ans = 0 )
{
    return i == str.size() ? ans :
               str[i] == c ? count(str, c, i + 1, ans + 1) :
                             count(str, c, i + 1, ans);
}

int main(int argc, char const *argv[])
{
    static_assert( count("dude", 'd') == 2, "d != 2" );
    return 0;
}

但是，博客文章中的代码未编译:

However, the code from the blog post is not compiling:

<代码>/usr/bin/clang ++ -Xclang -ast-print -fsyntax-only test_debugger2.cpp>main.exe

/usr/bin/clang++ -Xclang -ast-print -fsyntax-only test_debugger2.cpp > main.exe

test_debugger2.cpp:12:48: error: cannot initialize a member subobject of type 'char *const' with an lvalue of type 'const char [5]'
    constexpr StrWrap( const char(&arr)[N] ) : begin_(arr), size_(N - 1) {
                                               ^      ~~~
test_debugger2.cpp:38:26: note: in instantiation of function template specialization 'StrWrap::StrWrap<5>' requested here
    static_assert( count("dude", 'd') == 2, "d != 2" );
                         ^
test_debugger2.cpp:38:20: error: static_assert expression is not an integral constant expression
    static_assert( count("dude", 'd') == 2, "d != 2" );
                   ^~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

我可以找到与此相关的问题

I could find this related question Cannot initialize a member subobject of type 'const signed char *' with an lvalue of type 'const char [X]', but I did not get how I could fix it.

通过使用标志 -Xclang -ast-print -fsyntax-only ，我只要求 clang 扩展模板并将其扩展打印到标准输出:使用可变参数模板的编译时字符串"操作

By using the flags -Xclang -ast-print -fsyntax-only I am only asking clang to expand the templates and print their expansion to stdout: Compile-time 'String' Manipulation with Variadic Templates

我正在尝试学习使用C ++模板在编译时进行字符串计算，例如，字符串拆分，字符串连接，查找，替换，剪切等，该模板据说是图灵完成的:C++模板图灵完成?，然后，我以该博文中的示例为例，并使用 clang 查看它们如何工作以及如何使用它们来解决带有模板的任何编译时(可能)的计算.

I am trying to learn to do string computations, i.e., string splitting, string concatenation, find, replace, cut, etc, at compile time with C++ Templates which are said to be Turing Complete: C++ templates Turing-complete?, then, I am taking examples as that blog post and expanding them with clang to see how they work and how I could use them to solve any compile time (possible) computations with templates.

这是我的编译器版本:

clang version 8.0.1 (tags/RELEASE_801/final)
Target: x86_64-unknown-windows-cygnus
Thread model: posix
InstalledDir: /usr/bin

如何修改该博客文章中的模板扩展?

How could I fix the template expansion from that blog post?

推荐答案

此

char * const begin_;

是指向 char 的 const 指针(指针是const，而不是它指向的东西).这个

is a const pointer to char (the pointer is const, not the thing it points to). This

const char(&arr)[N]

是对类型为 const char 的 N 个元素的数组的引用.数组的元素是const.您不能将指向非const的指针指向const元素数组(至少不是没有蛮力).您需要将指针设为 const char * 或使数组成为非常量元素的数组……

is a reference to an array of N elements of type const char. The element of the array are const. You cannot have a pointer to non-const point into an array of const elements (at least not without brute force). You need to either make your pointer a const char* or make your array an array of non-const elements…

我想你是要写

const char* begin_;

注意:从C ++ 17开始，您可以仅使用 std:: string_view …

Note: starting with C++17, you can just use std::string_view…

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