Question

在C ++ 03中，当您使用运算符typeid时，会返回type_info个对象。

是否可以仅根据此结果检索给定类型的大小，例如由sizeof运算符返回？

例如：

std::type_info info = typeid(int);
int intSize = sizeof(int);
int intSize2 = info.getSize(); // doesn't exist!

问题是我们使用第三方多数组类来返回type_info，但不是类型的大小。

Answer 1

我能看到的最佳方式（我希望被证明是错误的）是预先注册类型，如下所示：

#include <typeinfo>
#include <iostream>
#include <stdexcept>
#include <map>
#include <vector>

typedef std::map<const std::type_info*, std::size_t> sizes_container; // we cannot use std::type_index, but that's ok - typeid returns const std::type_info&, which refers to an object which lives during the entire lifetime of the program

sizes_container& sizes() // wrapped in a function to avoid static initialization order fiasco
{
    static sizes_container s;
    return s;
}

template<typename T>
void register_size() // Register the type. Can be called multiple times for the same type.
{
    sizes().insert(sizes_container::value_type(&typeid(T), sizeof(T)));
}

class no_such_type : public std::domain_error
{
public:
    no_such_type(const std::string& str) :
        std::domain_error(str)
    {

    }
};

std::size_t get_size(const std::type_info& typeinfo)
{
    sizes_container::iterator it = sizes().find(&typeinfo);
    if(it != sizes().end())
    {
        return it->second;
    }
    else
    {
        throw no_such_type(std::string("No type ") + typeinfo.name() + " registered");
    }
}

int main()
{
    register_size<int>();
    register_size<std::vector<int> >();

    std::cout << get_size(typeid(int)) << "\n" // get the size from std::type_info, possibly at runtime
              << get_size(typeid(std::vector<int>)) << "\n" << std::flush;
    std::cout << get_size(typeid(long)); // if the type isn't registered, the exception no_such_type is thrown
}

可能的输出：

4
24

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
terminate called after throwing an instance of 'no_such_type'
  what():  No type l registered

如果您可以控制如何创建数组（例如，使用工厂方法），则可以在此处直接注册类型。

从std :: type_info检索数据类型的大小

1 个答案: