C++编译期循环获取变量类型
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文章目录
问题
假设现在有一些属性以及这些属性对应的数值类型,比如:
"gender" --> char
"age" --> int
"height" --> float
"IQ" ---> int
"name" --> std::string
"weight" --> double
在C++中,如何在编译期依次循环获取这些属性的数值类型,并根据对应的数值类型做出相应的处理(属性可能会增加)
二、解决方案
1.定义类型
首先把所有可能的类型用std::tuple列出:
using Types = std::tuple<float, int, double, std::string, char>;
template<std::size_t N>
using AttributeType = typename std::tuple_element<N, Types>::type;
这里,通过AttributeType<0>就可以得到float 类型
2.定义属性集
将所有的属性和其类型的对应关系列出,由于需要是编译期获得,必须类似加入constexpr 关键字:
constexpr const char* FLOAT_TYPE = "float";
constexpr const char* INT_TYPE = "int";
constexpr const char* DOUBLE_TYPE = "double";
constexpr const char* STRING_TYPE = "std::string";
constexpr const char* CHAR_TYPE = "float";
constexpr std::array<std::pair<const char*, const char*>, 6> attribute2type =
"gender", CHAR_TYPE,
"age", INT_TYPE,
"height", FLOAT_TYPE,
"IQ", INT_TYPE,
"name", STRING_TYPE,
"weight", DOUBLE_TYPE,
;
3. 获取类型索引
根据2中定义的类型字符串,获取1中需要的类型索引N:
constexpr std::size_t getTypeIndex(const char* name)
return strings_equal(name, "float") ? 0:
strings_equal(name, "int") ? 1:
strings_equal(name, "double") ? 2:
strings_equal(name, "std::string") ? 3:
strings_equal(name, "char") ? 4:
5; // compilation error
这里,需要一个编译期进行字符串比较的函数:
constexpr bool strings_equal(const char* a, const char* b)
return *a == *b && (*a == '\\0' || strings_equal(a + 1, b + 1));
4. 编译期循环
如何实现编译期的类似for循环呢,显然不能直接用for,模板的特性决定了可以使用编译期递归来进行替代for循环:
template <typename T>
void print(const char* attribute)
std::cout << "attribute = " << attribute << ",type=" << typeid(T).name() << std::endl;
constexpr size_t LAST_INDEX = attribute2type.size() - 1;
template <size_t T=LAST_INDEX>
struct PrintHelper
public:
PrintHelper()
doPrint<T>();
private:
template <size_t N>
void doPrint()
print<AttributeType<getTypeIndex(std::get<N>(attribute2type).second)>>(std::get<N>(attribute2type).first);
doPrint<N-1>();
;
template <>
template <>
void PrintHelper<LAST_INDEX>::doPrint<0>()
print<AttributeType<getTypeIndex(std::get<0>(attribute2type).second)>>(std::get<0>(attribute2type).first);
将上面所有的代码放到一块,就得到了一个可以在编译期循环获取变量类型的程序:
#include <string>
#include <iostream>
#include <typeinfo>
#include <tuple>
#include <array>
using Types = std::tuple<float, int, double, std::string, char>;
template<std::size_t N>
using AttributeType = typename std::tuple_element<N, Types>::type;
constexpr bool strings_equal(const char* a, const char* b)
return *a == *b && (*a == '\\0' || strings_equal(a + 1, b + 1));
constexpr std::size_t getTypeIndex(const char* name)
return strings_equal(name, "float") ? 0:
strings_equal(name, "int") ? 1:
strings_equal(name, "double") ? 2:
strings_equal(name, "std::string") ? 3:
strings_equal(name, "char") ? 4:
5; // compilation error
constexpr const char* FLOAT_TYPE = "float";
constexpr const char* INT_TYPE = "int";
constexpr const char* DOUBLE_TYPE = "double";
constexpr const char* STRING_TYPE = "std::string";
constexpr const char* CHAR_TYPE = "float";
constexpr std::array<std::pair<const char*, const char*>, 6> attribute2type =
"gender", CHAR_TYPE,
"age", INT_TYPE,
"height", FLOAT_TYPE,
"IQ", INT_TYPE,
"name", STRING_TYPE,
"weight", DOUBLE_TYPE,
;
template <typename T>
void print(const char* attribute)
std::cout << "attribute = " << attribute << ",type=" << typeid(T).name() << std::endl;
constexpr size_t LAST_INDEX = attribute2type.size() - 1;
template <size_t T=LAST_INDEX>
struct PrintHelper
public:
PrintHelper()
doPrint<T>();
private:
template <size_t N>
void doPrint()
print<AttributeType<getTypeIndex(std::get<N>(attribute2type).second)>>(std::get<N>(attribute2type).first);
doPrint<N-1>();
;
template <>
template <>
void PrintHelper<LAST_INDEX>::doPrint<0>()
print<AttributeType<getTypeIndex(std::get<0>(attribute2type).second)>>(std::get<0>(attribute2type).first);
int main()
PrintHelper<LAST_INDEX>();
return 0;
上面程序输出:
$ ./attributeWithType
attribute = weight,type=d
attribute = name,type=NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
attribute = IQ,type=i
attribute = height,type=f
attribute = age,type=i
attribute = gender,type=f
总结
本文通过下面几个技术点实现了编译期循环获取变量类型:
- 通过std::tuple定义变量类型集合,通过typename std::tuple_element<N, Types>::type获取某个变量类型
- 通过编译期递归实现编译期字符串比较
- 通过std::get(attribute2type)获取属性
- 编译期递归实现循环获取变量类型
Reference
- https://stackoverflow.com/questions/27490858/how-can-you-compare-two-character-strings-statically-at-compile-time
- https://stackoverflow.com/questions/55183295/c-mapping-a-set-of-values-to-types
- https://stackoverflow.com/questions/6872919/compile-time-loops
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