与其他类似的问题不同，这个问题是关于如何使用c++的新特性。
2008 c Is there a simple way to convert C++ enum to string?
2008 c Easy way to use variables of enum types as string in C?
2008 c++ How to easily map c++ enums to strings
2008 c++ Making something both a C identifier and a string?
2008 c++ Is there a simple script to convert C++ enum to string?
2009 c++ How to use enums as flags in C++?
2011 c++ How to convert an enum type variable to a string?
2011 c++ Enum to String C++
2011 c++ How to convert an enum type variable to a string?
2012 c How to convert enum names to string in c
2013 c Stringifying an conditionally compiled enum in C
看了很多答案后，我还没有找到:
优雅的方式使用c++ 11、c++ 14或c++ 17的新特性
或者在Boost中使用一些现成的东西
还有一些东西计划在c++ 20中实现
例子
举例往往比冗长的解释更好。
您可以在Coliru上编译和运行这个代码片段。
(另一个前面的例子也可用)
#include <map>
#include <iostream>
struct MyClass
{
enum class MyEnum : char {
AAA = -8,
BBB = '8',
CCC = AAA + BBB
};
};
// Replace magic() by some faster compile-time generated code
// (you're allowed to replace the return type with std::string
// if that's easier for you)
const char* magic (MyClass::MyEnum e)
{
const std::map<MyClass::MyEnum,const char*> MyEnumStrings {
{ MyClass::MyEnum::AAA, "MyClass::MyEnum::AAA" },
{ MyClass::MyEnum::BBB, "MyClass::MyEnum::BBB" },
{ MyClass::MyEnum::CCC, "MyClass::MyEnum::CCC" }
};
auto it = MyEnumStrings.find(e);
return it == MyEnumStrings.end() ? "Out of range" : it->second;
}
int main()
{
std::cout << magic(MyClass::MyEnum::AAA) <<'\n';
std::cout << magic(MyClass::MyEnum::BBB) <<'\n';
std::cout << magic(MyClass::MyEnum::CCC) <<'\n';
}
约束
请不要无价值的重复其他答案或基本链接。
请避免基于宏的臃肿答案，或尽量减少#define开销。
请不要手动enum ->字符串映射。
很高兴有
支持从不同于零的数字开始的enum值
支持负enum值
支持碎片enum值
支持类枚举(c++ 11)
支持类枚举:<类型>有任何允许的<类型> (c++ 11)
编译时(不是运行时)到字符串的转换，
或者至少在运行时快速执行(例如std::map不是一个好主意…)
constexpr (c++ 11，然后在c++ 14/17/20中放松)
noexcept (C + + 11)
c++ 17/ c++ 20友好的代码片段
一个可能的想法是使用c++编译器功能，在编译时使用基于可变参数模板类和constexpr函数的元编程技巧来生成c++代码……
嗯，还有另一个选择。一个典型的用例是，您需要为HTTP谓词使用常量，并使用其字符串版本值。
示例:
int main () {
VERB a = VERB::GET;
VERB b = VERB::GET;
VERB c = VERB::POST;
VERB d = VERB::PUT;
VERB e = VERB::DELETE;
std::cout << a.toString() << std::endl;
std::cout << a << std::endl;
if ( a == VERB::GET ) {
std::cout << "yes" << std::endl;
}
if ( a == b ) {
std::cout << "yes" << std::endl;
}
if ( a != c ) {
std::cout << "no" << std::endl;
}
}
VERB类:
// -----------------------------------------------------------
// -----------------------------------------------------------
class VERB {
private:
// private constants
enum Verb {GET_=0, POST_, PUT_, DELETE_};
// private string values
static const std::string theStrings[];
// private value
const Verb value;
const std::string text;
// private constructor
VERB (Verb v) :
value(v), text (theStrings[v])
{
// std::cout << " constructor \n";
}
public:
operator const char * () const { return text.c_str(); }
operator const std::string () const { return text; }
const std::string toString () const { return text; }
bool operator == (const VERB & other) const { return (*this).value == other.value; }
bool operator != (const VERB & other) const { return ! ( (*this) == other); }
// ---
static const VERB GET;
static const VERB POST;
static const VERB PUT;
static const VERB DELETE;
};
const std::string VERB::theStrings[] = {"GET", "POST", "PUT", "DELETE"};
const VERB VERB::GET = VERB ( VERB::Verb::GET_ );
const VERB VERB::POST = VERB ( VERB::Verb::POST_ );
const VERB VERB::PUT = VERB ( VERB::Verb::PUT_ );
const VERB VERB::DELETE = VERB ( VERB::Verb::DELETE_ );
// end of file
对于c++ 17 c++ 20，您将对反思研究小组(SG7)的工作感兴趣。还有一系列平行的论文，包括措辞(P0194)和基本原理、设计和进化(P0385)。(链接解析为每个系列的最新论文。)
从P0194r2(2016-10-15)开始，该语法将使用建议的reflexpr关键字:
meta::get_base_name_v<
meta::get_element_m<
meta::get_enumerators_m<reflexpr(MyEnum)>,
0>
>
例如(改编自Matus Choclik的reflexpr clang分支):
#include <reflexpr>
#include <iostream>
enum MyEnum { AAA = 1, BBB, CCC = 99 };
int main()
{
auto name_of_MyEnum_0 =
std::meta::get_base_name_v<
std::meta::get_element_m<
std::meta::get_enumerators_m<reflexpr(MyEnum)>,
0>
>;
// prints "AAA"
std::cout << name_of_MyEnum_0 << std::endl;
}
静态反射未能进入c++ 17(更确切地说，进入了2016年11月在Issaquah举行的标准会议上提出的可能是最终草案)，但有信心它将进入c++ 20;摘自赫布·萨特的旅行报告:
特别是，反射研究小组审查了最新合并的静态反射提案，并发现它准备在我们的下一次会议上进入主要的进化小组，开始考虑TS或下一个标准的统一静态反射提案。
嗯，还有另一个选择。一个典型的用例是，您需要为HTTP谓词使用常量，并使用其字符串版本值。
示例:
int main () {
VERB a = VERB::GET;
VERB b = VERB::GET;
VERB c = VERB::POST;
VERB d = VERB::PUT;
VERB e = VERB::DELETE;
std::cout << a.toString() << std::endl;
std::cout << a << std::endl;
if ( a == VERB::GET ) {
std::cout << "yes" << std::endl;
}
if ( a == b ) {
std::cout << "yes" << std::endl;
}
if ( a != c ) {
std::cout << "no" << std::endl;
}
}
VERB类:
// -----------------------------------------------------------
// -----------------------------------------------------------
class VERB {
private:
// private constants
enum Verb {GET_=0, POST_, PUT_, DELETE_};
// private string values
static const std::string theStrings[];
// private value
const Verb value;
const std::string text;
// private constructor
VERB (Verb v) :
value(v), text (theStrings[v])
{
// std::cout << " constructor \n";
}
public:
operator const char * () const { return text.c_str(); }
operator const std::string () const { return text; }
const std::string toString () const { return text; }
bool operator == (const VERB & other) const { return (*this).value == other.value; }
bool operator != (const VERB & other) const { return ! ( (*this) == other); }
// ---
static const VERB GET;
static const VERB POST;
static const VERB PUT;
static const VERB DELETE;
};
const std::string VERB::theStrings[] = {"GET", "POST", "PUT", "DELETE"};
const VERB VERB::GET = VERB ( VERB::Verb::GET_ );
const VERB VERB::POST = VERB ( VERB::Verb::POST_ );
const VERB VERB::PUT = VERB ( VERB::Verb::PUT_ );
const VERB VERB::DELETE = VERB ( VERB::Verb::DELETE_ );
// end of file
#define ENUM_MAKE(TYPE, ...) \
enum class TYPE {__VA_ARGS__};\
struct Helper_ ## TYPE { \
static const String& toName(TYPE type) {\
int index = static_cast<int>(type);\
return splitStringVec()[index];}\
static const TYPE toType(const String& name){\
static std::unordered_map<String,TYPE> typeNameMap;\
if( typeNameMap.empty() )\
{\
const StringVector& ssVec = splitStringVec();\
for (size_t i = 0; i < ssVec.size(); ++i)\
typeNameMap.insert(std::make_pair(ssVec[i], static_cast<TYPE>(i)));\
}\
return typeNameMap[name];}\
static const StringVector& splitStringVec() {\
static StringVector typeNameVector;\
if(typeNameVector.empty()) \
{\
typeNameVector = StringUtil::split(#__VA_ARGS__, ",");\
for (auto& name : typeNameVector)\
{\
name.erase(std::remove(name.begin(), name.end(), ' '),name.end()); \
name = String(#TYPE) + "::" + name;\
}\
}\
return typeNameVector;\
}\
};
using String = std::string;
using StringVector = std::vector<String>;
StringVector StringUtil::split( const String& str, const String& delims, unsigned int maxSplits, bool preserveDelims)
{
StringVector ret;
// Pre-allocate some space for performance
ret.reserve(maxSplits ? maxSplits+1 : 10); // 10 is guessed capacity for most case
unsigned int numSplits = 0;
// Use STL methods
size_t start, pos;
start = 0;
do
{
pos = str.find_first_of(delims, start);
if (pos == start)
{
// Do nothing
start = pos + 1;
}
else if (pos == String::npos || (maxSplits && numSplits == maxSplits))
{
// Copy the rest of the string
ret.push_back( str.substr(start) );
break;
}
else
{
// Copy up to delimiter
ret.push_back( str.substr(start, pos - start) );
if(preserveDelims)
{
// Sometimes there could be more than one delimiter in a row.
// Loop until we don't find any more delims
size_t delimStart = pos, delimPos;
delimPos = str.find_first_not_of(delims, delimStart);
if (delimPos == String::npos)
{
// Copy the rest of the string
ret.push_back( str.substr(delimStart) );
}
else
{
ret.push_back( str.substr(delimStart, delimPos - delimStart) );
}
}
start = pos + 1;
}
// parse up to next real data
start = str.find_first_not_of(delims, start);
++numSplits;
} while (pos != String::npos);
return ret;
}
例子
ENUM_MAKE(MY_TEST, MY_1, MY_2, MY_3)
MY_TEST s1 = MY_TEST::MY_1;
MY_TEST s2 = MY_TEST::MY_2;
MY_TEST s3 = MY_TEST::MY_3;
String z1 = Helper_MY_TEST::toName(s1);
String z2 = Helper_MY_TEST::toName(s2);
String z3 = Helper_MY_TEST::toName(s3);
MY_TEST q1 = Helper_MY_TEST::toType(z1);
MY_TEST q2 = Helper_MY_TEST::toType(z2);
MY_TEST q3 = Helper_MY_TEST::toType(z3);
自动ENUM_MAKE宏生成“枚举类”和“枚举反射函数”辅助类。
为了减少错误，Everything只定义一个ENUM_MAKE。
这种代码的优点是自动创建用于反射和查看宏代码，易于理解的代码。'enum to string'， 'string to enum'性能都是算法O(1)。
缺点是第一次使用时，枚举relection的string vector和map的helper类被初始化。
但是如果你想，你也会被预初始化。- - - - - -
如果你的枚举看起来像
enum MyEnum
{
AAA = -8,
BBB = '8',
CCC = AAA + BBB
};
你可以移动枚举的内容到一个新文件:
AAA = -8,
BBB = '8',
CCC = AAA + BBB
然后这些值可以被宏包围:
// default definition
#ifned ITEM(X,Y)
#define ITEM(X,Y)
#endif
// Items list
ITEM(AAA,-8)
ITEM(BBB,'8')
ITEM(CCC,AAA+BBB)
// clean up
#undef ITEM
下一步可能是重新包含枚举中的项:
enum MyEnum
{
#define ITEM(X,Y) X=Y,
#include "enum_definition_file"
};
最后你可以生成关于这个枚举的实用函数:
std::string ToString(MyEnum value)
{
switch( value )
{
#define ITEM(X,Y) case X: return #X;
#include "enum_definition_file"
}
return "";
}
MyEnum FromString(std::string const& value)
{
static std::map<std::string,MyEnum> converter
{
#define ITEM(X,Y) { #X, X },
#include "enum_definition_file"
};
auto it = converter.find(value);
if( it != converter.end() )
return it->second;
else
throw std::runtime_error("Value is missing");
}
该解决方案可以应用于旧的c++标准，它不使用现代的c++元素，但它可以用来生成大量代码，而不需要太多的工作和维护。
这个要点提供了一个基于c++可变参数模板的简单映射。
这是一个c++ 17简化版的基于类型的映射的要点:
#include <cstring> // http://stackoverflow.com/q/24520781
template<typename KeyValue, typename ... RestOfKeyValues>
struct map {
static constexpr typename KeyValue::key_t get(const char* val) noexcept {
if constexpr (sizeof...(RestOfKeyValues)==0) // C++17 if constexpr
return KeyValue::key; // Returns last element
else {
static_assert(KeyValue::val != nullptr,
"Only last element may have null name");
return strcmp(val, KeyValue::val())
? map<RestOfKeyValues...>::get(val) : KeyValue::key;
}
}
static constexpr const char* get(typename KeyValue::key_t key) noexcept {
if constexpr (sizeof...(RestOfKeyValues)==0)
return (KeyValue::val != nullptr) && (key == KeyValue::key)
? KeyValue::val() : "";
else
return (key == KeyValue::key)
? KeyValue::val() : map<RestOfKeyValues...>::get(key);
}
};
template<typename Enum, typename ... KeyValues>
class names {
typedef map<KeyValues...> Map;
public:
static constexpr Enum get(const char* nam) noexcept {
return Map::get(nam);
}
static constexpr const char* get(Enum key) noexcept {
return Map::get(key);
}
};
用法示例:
enum class fasion {
fancy,
classic,
sporty,
emo,
__last__ = emo,
__unknown__ = -1
};
#define NAME(s) static inline constexpr const char* s() noexcept {return #s;}
namespace name {
NAME(fancy)
NAME(classic)
NAME(sporty)
NAME(emo)
}
template<auto K, const char* (*V)()> // C++17 template<auto>
struct _ {
typedef decltype(K) key_t;
typedef decltype(V) name_t;
static constexpr key_t key = K; // enum id value
static constexpr name_t val = V; // enum id name
};
typedef names<fasion,
_<fasion::fancy, name::fancy>,
_<fasion::classic, name::classic>,
_<fasion::sporty, name::sporty>,
_<fasion::emo, name::emo>,
_<fasion::__unknown__, nullptr>
> fasion_names;
map < keyvalue…>可以双向使用:
fasion_names:把(in fashion: emo)
fasion_names::把(“emo”)
这个例子可以在godbolt.org上找到
int main ()
{
constexpr auto str = fasion_names::get(fasion::emo);
constexpr auto fsn = fasion_names::get(str);
return (int) fsn;
}
结果:gc -7 -std=c++1z -Ofast -S
main:
mov eax, 3
ret
