C++编译期循环获取变量类型

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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）

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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 = "char";

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 = "char";

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=c

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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：

1. 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
2. 通过编译期递归实现编译期字符串比较
3. 通过std::get(attribute2type)获取属性
4. 编译期递归实现循环获取变量类型

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Reference

1. https://stackoverflow.com/questions/27490858/how-can-you-compare-two-character-strings-statically-at-compile-time
2. https://stackoverflow.com/questions/55183295/c-mapping-a-set-of-values-to-types
3. https://stackoverflow.com/questions/6872919/compile-time-loops