在编译时生成简单格式字符串
我试图在编译时为 fmt 生成一个简单的格式字符串,但我无法完全弄清楚字符串连接。我仅限于 c++ 14。
我想要的是能够为 N 个项目生成格式字符串,因此可以按如下方式使用它: auto my_string = fmt::format(format_string_struct<2>::format_string, "Item", "key1", "value1", "key2", "value2");
生成的格式字符串将为:“{} {}={} {}={}”,生成“Item key1=value1 key2=value2”的格式化字符串。格式字符串的基础是“{}”,并且它为每个 N 附加一个“{}={}”。
我正在尝试使用递归模板,但我无法完全让一切正常工作。我从 连接编译时字符串中得到了一些代码编译时的模板? 用于连接字符串(尽管我得到了对 `concat_impl<&base_format_string_struct::format_string 的 未定义引用, std::integer_sequence
template<int...I> using is = std::integer_sequence<int,I...>;
template<int N> using make_is = std::make_integer_sequence<int,N>;
constexpr auto size(const char*s) { int i = 0; while(*s!=0){++i;++s;} return i; }
template<const char*, typename, const char*, typename>
struct concat_impl;
template<const char* S1, int... I1, const char* S2, int... I2>
struct concat_impl<S1, is<I1...>, S2, is<I2...>> {
static constexpr const char value[]
{
S1[I1]..., S2[I2]..., 0
};
};
template<const char* S1, const char* S2>
constexpr auto concat {
concat_impl<S1, make_is<size(S1)>, S2, make_is<size(S2)>>::value
};
struct base_format_string_struct {
static constexpr const char format_string[] = "{}";
};
struct format_string_parameter_struct {
static constexpr const char parameter_string[] = " {}=\"{}\"";
};
template <int arg_count, const char[]... params>
struct format_string_struct:
public format_string_struct<arg_count - 1, format_string_parameter_struct, params...> {
};
template <int arg_count>
struct format_string_struct:
public format_string_struct<arg_count - 1, format_string_parameter_struct> {
};
template <const char[]... params>
struct format_string_struct<0> {
static const char* format_string() {
return concat<base_format_string_struct, params...>;
}
};
I'm trying to generate a simple format string for fmt at compile time, but I can't quite figure out the string concatenation. I'm limited to c++ 14.
What I'd like is to be able to generate a format string for N items, so it could be used as follows:auto my_string = fmt::format(format_string_struct<2>::format_string, "Item", "key1", "value1", "key2", "value2");
The generated format string would be: "{} {}={} {}={}", resulting in a formatted string of "Item key1=value1 key2=value2". The base of the format string would be "{}", and it appends a " {}={}" for each N.
I'm trying to use recursive templates, but I just can't quite get everything to work. I got some code from Concatenate compile-time strings in a template at compile time? for concatenating strings (though I get a undefined reference to `concat_impl<&base_format_string_struct::format_string, std::integer_sequence<int, 0, 1>, &format_string_parameter_struct::parameter_string, std::integer_sequence<int, 0, 1, 2, 3, 4, 5, 6, 7> >::value'
error)
template<int...I> using is = std::integer_sequence<int,I...>;
template<int N> using make_is = std::make_integer_sequence<int,N>;
constexpr auto size(const char*s) { int i = 0; while(*s!=0){++i;++s;} return i; }
template<const char*, typename, const char*, typename>
struct concat_impl;
template<const char* S1, int... I1, const char* S2, int... I2>
struct concat_impl<S1, is<I1...>, S2, is<I2...>> {
static constexpr const char value[]
{
S1[I1]..., S2[I2]..., 0
};
};
template<const char* S1, const char* S2>
constexpr auto concat {
concat_impl<S1, make_is<size(S1)>, S2, make_is<size(S2)>>::value
};
struct base_format_string_struct {
static constexpr const char format_string[] = "{}";
};
struct format_string_parameter_struct {
static constexpr const char parameter_string[] = " {}=\"{}\"";
};
template <int arg_count, const char[]... params>
struct format_string_struct:
public format_string_struct<arg_count - 1, format_string_parameter_struct, params...> {
};
template <int arg_count>
struct format_string_struct:
public format_string_struct<arg_count - 1, format_string_parameter_struct> {
};
template <const char[]... params>
struct format_string_struct<0> {
static const char* format_string() {
return concat<base_format_string_struct, params...>;
}
};
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您有几个错别字(限制为2个参数)。
demo
,但我可能会使用constexpr函数:
demo
You have several typos (and a concat limited to 2 argument).
Demo
But I would probably go with constexpr functions:
Demo