support bazel complie this project and format code.

cpp2.0/cpp11/decltype.cpp

@@ -0,0 +1,56 @@
+//
+// Created by light on 19-11-3.
+//
+
+#include <iostream>
+#include <map>
+#include <set>
+
+using namespace std;
+
+// 1.used to declare return tyoes  返回类型
+
+// c++2.0之前编译不出来
+// template<typename T1, typename T2>
+// decltype(x + y) add(T1 x, T2 y);
+
+// 编译器编不出来在2.0之后变成下面这个,先用auto暂定,后面返回类型为decltype(x+y)
+template <typename T1, typename T2>
+auto add(T1 x, T2 y) -> decltype(x + y)
+{ // -> 也用在lambda
+    return x + y;
+}
+
+class Person
+{
+public:
+    string firstname;
+    string lastname;
+};
+int main()
+{
+
+    // 1.used to declare return tyoes
+    cout << add(1, 2) << endl;
+
+    // 2.模板元编程 例如在一个模板函数或类获取容器的value_type,这里就不封装了,直接写在main函数里面
+    // 获得表达式的type 有点像typeof()特点
+    map<string, float> coll;
+    // 获取上述类型
+    decltype(coll)::value_type m{"as", 1}; // value_type为pair<string,int> m
+    cout << m.first << " " << m.second << endl;
+    pair<string, int> p{"a", 2};
+    cout << p.first << " " << p.second << endl;
+    // 3.used to pass the type of a lambda
+
+    // 比大小
+    auto cmp = [](const Person &p1, const Person &p2)
+    {
+        return p1.lastname < p2.lastname;
+    };
+
+    // 对于lambda,我们往往只有object,很少有人能够写出它的类型，而有时就需要知道它的类型,要获得其type,就要借助其decltype
+    set<Person, decltype(cmp)> col(cmp);
+
+    return 0;
+}