如何在编译时将C字符串转换为int? [英] How do I convert a C string to a int at compile time?
问题描述
我希望能够传递整数或双精度(或字符串)作为模板参数,在某些情况下将结果转换为整数,并将其用作模板参数
I want to be able to pass an integer or a double (or a string) as a template argument and in some instances convert the result to an integer and use it as a template argument for a type in the class.
这是我尝试过的内容:
template <typename MPLString>
class A
{
// the following works fine
int fun()
{
// this function should return the int in the boost mpl type passed to it
// (e.g. it might be of the form "123")
return std::stoi(boost::mpl::c_str<MPLString>::value);
}
// the following breaks because std::stoi is not constexpr
std::array<int, std::stoi(boost::mpl::c_str<MPLString>::value)> arr;
};
我能以某种方式这样做吗?我试过 std :: stoi
和 atoi
,但都不是 constexpr
...怎么做(我不能将模板参数更改为直接采用 int
的任何想法)
Can I do this somehow? I've tried std::stoi
and atoi
but neither are constexpr
... Any ideas how this could be done (I cannot change the template parameter to take an int
directly, as it might be a double).
推荐答案
定义 constexpr stoi
不太合适使用常规C字符串很难。可以定义如下:
Defining a constexpr stoi
isn't too hard with regular C strings. It can be defined as follows:
constexpr bool is_digit(char c) {
return c <= '9' && c >= '0';
}
constexpr int stoi_impl(const char* str, int value = 0) {
return *str ?
is_digit(*str) ?
stoi_impl(str + 1, (*str - '0') + value * 10)
: throw "compile-time-error: not a digit"
: value;
}
constexpr int stoi(const char* str) {
return stoi_impl(str);
}
int main() {
static_assert(stoi("10") == 10, "...");
}
在常量表达式中使用throw表达式无效时,它将触发一个编译时错误,而不是实际抛出。
The throw expression is invalid when used in constant expressions so it'll trigger a compile time error rather than actually throwing.
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