024 Structured bindings

Jason在这期里简单介绍了structured binding在C++17中的运用，挺有意思。其实我在想，要是structured binding可以忽略一些变量就好了，就像是Python里的下划线“_”的作用。目前可以通过std::tie和std::ignore来实现类似的功能，但是要预先定义变量。

Structured bindings除了用在std::pair和std::tuple上之外，用在普通的array和std::array上也是可以的。

Jason同时演示了structured binding可以用在struct的成员变量上。但是当struct/class具有继承关系时，structured bindings就基本失效了。当一个class具有私有成员变量时也不行。可以看的讨论。

最后需要指出的是，在使用structured bindings时，注意auto和auto&的差别。

#include 
   
    #include 
    
     #include 
     #include 
      
       #include 
       
        struct A {
       int    a = 0;    float  b = 1.1f;    double c = 2.2;    std::string d = "d";};class B {
   public:    int a = 0;    int b = 1.1f;private:    int c = 2.2;};class C {
   public:    int a = 0;    int b = 1;public:    void f() {
   }private:    void g() {
   }public:    int c = 2;};class D {
   public:    int d = 3;};class E0 : public C {
   public:    int e = 4;};class E1 : public C, D {
   public:    int e = 4;};class F {
   public:    static int a;};int F::a = 0;class G : public F {
   public:    static int b;};int G::b = 0;int& add_v( std::map
        
         & v, const std::string& name) { if ( auto [ iter, flag ] = v.insert({ name, 0}); flag ) { return iter->second; } else { throw std::runtime_error("Add failed. "); }}int main() { std::cout << "Hello, StructuredBinding! \n"; std::map
         
           m; auto& ret = add_v(m, "v0"); std::cout << "m[\"v0\"] = " << m["v0"] << '\n'; ret = 1; std::cout << "m[\"v0\"] = " << m["v0"] << '\n'; std::cout << '\n'; // Test std::tie and std::ignore. bool flag = false; std::tie( std::ignore, flag ) = m.insert({ "v1", 0}); std::cout << "Test with plain array and std::array. \n"; { int a[] = { 0, 1, 2}; auto [ a0, a1, a2 ] = a; std::array b = { 0, 1, 2 }; auto [ b0, b1, b2 ] = b; } std::cout << "Test structured binding with structs and classes. \n"; { auto [ a, b, c, d ] = A(); std::cout << "a = " << a << '\n'; std::cout << "b = " << b << '\n'; std::cout << "c = " << c << '\n'; std::cout << "d = " << d << '\n'; // Decompose A into 2 elements. // auto [ aa, bb ] = A(); // This is an error. } // Test structured binding with class. { // Structured binding with privatre data members. // auto [ a, b ] = B(); // This is an error. auto [ a, b, c ] = C(); C objC; auto [ ca, cb, cc ] = objC; ca = -1; std::cout << "objC.a = " << objC.a << '\n'; auto & [ rca, rcb, rcc ] = objC; rca = -2; std::cout << "objC.a = " << objC.a << '\n'; } // Test structured binding with class hierarchy. { // auto [ a, b, e ] = E0(); // This is an error. // auto [ a, b, d, e ] = E1(); // This is an error. // auto [ a, b ] = G(); // This is an error. } return 0;}

上述代码的执行结果如下

Hello, StructuredBinding! m["v0"] = 0m["v0"] = 1Test with plain array and std::array. Test structured binding with structs and classes. a = 0b = 1.1c = 2.2d = dobjC.a = 0objC.a = -2

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