请看下面元函数_if:

template <bool B, typename L, typename R>
struct _if
{
    typedef R type;
};

template <typename L, typename R>
struct _if<true, L, R>
{ 
    typedef L type;
};
 

执行以下代码（1）：

#include<iostream>

/// <summary>
/// _if结构
/// </summary>
/// <typeparam name="L"></typeparam>
/// <typeparam name="R"></typeparam>
/// <typeparam name="B"></typeparam>
template <bool B, typename L, typename R>
struct _if
{
	typedef R type;

	_if()
	{
		std::cout << "结构1" << "\n";
	}
};

 

void main()
{
	//std::string var1 = "text"
	_if<true, int, std::string>::type var1 = "text";

	_if<true, int, std::string> a1;

}

输出：

现在上面代码中再加一个_if结构2

/// <summary>
/// _if结构2
/// </summary>
/// <typeparam name="L"></typeparam>
/// <typeparam name="R"></typeparam>
template <typename L, typename R>
struct _if<true, L, R>
{
    typedef L type;
};

 

/// <summary>
/// _if结构
/// </summary>
/// <typeparam name="L"></typeparam>
/// <typeparam name="R"></typeparam>
/// <typeparam name="B"></typeparam>
template <bool B, typename L, typename R>
struct _if
{
	typedef R type;

	_if()
	{
		std::cout << "结构1" << "\n";
	}
};

/// <summary>
/// i_if结构2
/// </summary>
/// <typeparam name="L"></typeparam>
/// <typeparam name="R"></typeparam>
template <typename L, typename R>
struct _if<true, L, R>
{
	typedef L type;
};




void main()
{
	//std::string var1 = "text"
	_if<true, int, std::string>::type var1 = "text";

	_if<true, int, std::string> a1;

}

执行结果：

为什么编译不过，因为现在用的是结构：

也就是说：_if<true, int, std::string>::type var1 => int (返回整型），修改代码：

#include<iostream>

/// <summary>
/// _if结构
/// </summary>
/// <typeparam name="L"></typeparam>
/// <typeparam name="R"></typeparam>
/// <typeparam name="B"></typeparam>
template <bool B, typename L, typename R>
struct _if
{
	typedef R type;

	_if()
	{
		std::cout << "结构1" << "\n";
	}
};

/// <summary>
/// i_if结构2
/// </summary>
/// <typeparam name="L"></typeparam>
/// <typeparam name="R"></typeparam>
template <typename L, typename R>
struct _if<true, L, R>
{
	typedef L type;
	_if()
	{
		std::cout << "结构2" << "\n";
	}
};




void main()
{ 
	_if<true, int, std::string>::type var1 = 5;
	_if<false, int, std::string>::type var2 = "text";

	_if<true, int, std::string> a1;
	_if<false, int, std::string> a2;
}

输出：

这就是元函数_if原理：

 _if 原函数：

template <bool B, typename L, typename R>
struct _if{	typedef R type;};

template <typename L, typename R>
struct _if<true, L, R>{ 	typedef L type;};