一文详解C++11中auto的使用

#include "auto.hpp"
#include <iostream>
#include <cmath>
#include <typeinfo>
#include <string>
#include <map>
#include <list>
#include <deque>
#include <vector>
//
// reference: http://en.cppreference.com/w/cpp/language/auto
template<class T, class U>
auto add(T t, U u) -> decltype(t + u) // the return type is the type of operator+(T, U)
{
	return t + u;
}
auto get_fun(int arg) -> double(*)(double) // same as: double (*get_fun(int))(double)
{
	switch (arg) {
		case 1: return std::fabs;
		case 2: return std::sin;
		default: return std::cos;
	}
}
int test_auto1()
{
	auto a = 1 + 2;
	std::cout << "type of a: " << typeid(a).name() << '\n'; // type of a: int
	auto b = add(1, 1.2);
	std::cout << "type of b: " << typeid(b).name() << '\n'; // type of b: double
	auto c = { 1, 2 };
	std::cout << "type of c: " << typeid(c).name() << '\n'; // type of c: class std::initializer_list<int>
	auto my_lambda = [](int x) { return x + 3; };
	std::cout << "my_lambda: " << my_lambda(5) << '\n'; // my_lambda: 8
	auto my_fun = get_fun(2);
	std::cout << "type of my_fun: " << typeid(my_fun).name() << '\n'; // type of my_fun: double (__cdecl*)(double)
	std::cout << "my_fun: " << my_fun(3) << '\n'; // my_fun: 0.14112
	// auto int x; // error as of C++11: "auto" is no longer a storage-class specifier // error C3530: “auto”不能与任何其他类型说明符组合
	return 0;
}
// reference: https://msdn.microsoft.com/zh-cn/library/dd293667(v=vs.120).aspx
int f(int x) { return x; }
int test_auto2()
{
	int count = 10;
	int& countRef = count;
	auto myAuto = countRef;
	countRef = 11;
	std::cout << count << " " << std::endl; // 11
	myAuto = 12;
	std::cout << count << std::endl; // 11
	// 1. 下面的声明等效。 在第一个语句中，声明 j 变量为类型 int。 在第二个语句，因为初始化表达式 (0) 是整数，所以变量 k 推导为 int 类型
	int j = 0;  // Variable j is explicitly type int.
	auto k = 0; // Variable k is implicitly type int because 0 is an integer.
	// 2. 以下声明等效，但第二个声明比第一个简单
	std::map<int, std::list<std::string>> m;
	std::map<int, std::list<std::string>>::iterator i = m.begin();
	auto i_ = m.begin();
	// 3. 声明 iter 和 elem 变量类型
	std::deque<double> dqDoubleData(10, 0.1);
	for (auto iter = dqDoubleData.begin(); iter != dqDoubleData.end(); ++iter) { /* ... */}
	// prefer range-for loops with the following information in mind
	// (this applies to any range-for with auto, not just deque)
	for (auto elem : dqDoubleData) // COPIES elements, not much better than the previous examples 
	{ /* ... */ }
	for (auto& elem : dqDoubleData) // observes and/or modifies elements IN-PLACE
	{ /* ... */ }
	for (const auto& elem : dqDoubleData) // observes elements IN-PLACE
	{ /* ... */ }
	// 4. 使用 new 运算符
	double x = 12.34;
	auto *y = new auto(x), **z = new auto(&x);
	// 5. 所有符号解析为同一类型
	auto x_ = 1, *y_ = &x_, **z_ = &y_; // Resolves to int.
	auto a(2.01), *b(&a);         // Resolves to double.
	auto c = 'a', *d(&c);          // Resolves to char.
	auto m_ = 1, &n_ = m_;            // Resolves to int.
	// 6. 使用条件运算符 (?:)
	int v1 = 100, v2 = 200;
	auto e = v1 > v2 ? v1 : v2;
	// 7. 将变量 x7 初始化类型 int，将引用的变量 y7 初始化为类型 const int，及将变量 fp 初始化为指向返回类型 int 的函数的指针
	auto x7 = f(0);
	const auto & y7 = f(1);
	int(*p)(int x7);
	p = f;
	auto fp = p;
	return 0;
}
/
// reference: http://www.learncpp.com/cpp-tutorial/4-8-the-auto-keyword/
int add_3(int x, int y)
{
	return x + y;
}
int test_auto3()
{
	auto d = 5.0; // 5.0 is a double literal, so d will be type double
	auto i = 1 + 2; // 1 + 2 evaluates to an integer, so i will be type int
	auto sum = add_3(5, 6); // add_3() returns an int, so sum will be type int
	return 0;
}