C++中类型推断(auto和decltype)的使用

// C++ program to demonstrate working of auto
// and type inference

#include <bits/stdc++.h>
using namespace std;

int main()
{
	// auto a; this line will give error
	// because 'a' is not initialized at
	// the time of declaration
	// a=33;

	// see here x ,y,ptr are
	// initialised at the time of
	// declaration hence there is
	// no error in them
	auto x = 4;
	auto y = 3.37;
	auto z = 3.37f;
	auto c = 'a';
	auto ptr = &x;
	auto pptr = &ptr; //pointer to a pointer
	cout << typeid(x).name() << endl
		<< typeid(y).name() << endl
		<< typeid(z).name() << endl
		<< typeid(c).name() << endl
		<< typeid(ptr).name() << endl
		<< typeid(pptr).name() << endl;

	return 0;
}

// C++ program to demonstrate that we can use auto to
// save time when creating iterators

#include <bits/stdc++.h>
using namespace std;

int main()
{
	// Create a set of strings
	set<string> st;
	st.insert({ "geeks", "for", "geeks", "org" });

	// 'it' evaluates to iterator to set of string
	// type automatically
	for (auto it = st.begin(); it != st.end(); it++)
		cout << *it << " ";

	return 0;
}

// C++ program to demonstrate use of decltype
#include <bits/stdc++.h>
using namespace std;

int fun1() { return 10; }
char fun2() { return 'g'; }

int main()
{
	// Data type of x is same as return type of fun1()
	// and type of y is same as return type of fun2()
	decltype(fun1()) x;
	decltype(fun2()) y;

	cout << typeid(x).name() << endl;
	cout << typeid(y).name() << endl;

	return 0;
}

// C++ program to demonstrate use of decltype
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{
	int x = 5;

	// j will be of type int : data type of x
	decltype(x) j = x + 5;

	cout << typeid(j).name();

	return 0;
}

// C++ program to demonstrate use of decltype in functions
#include <bits/stdc++.h>
using namespace std;

// A generic function which finds minimum of two values
// return type is type of variable which is minimum
template <class A, class B>
auto findMin(A a, B b) -> decltype(a < b ? a : b)
{
	return (a < b) ? a : b;
}

// driver function to test various inference
int main()
{
	// This call returns 3.44 of double type
	cout << findMin(4, 3.44) << endl;

	// This call returns 3 of double type
	cout << findMin(5.4, 3) << endl;

	return 0;
}