使用auto宣告，縮短程式

Example

map<int, string> data;

map<int, string>::iterator it = data.begin();  // OK
auto it = data.begin();                        // better

unsigned int i = data.size();  // OK
auto i = data.size();          // better, i 是 map<int, string>::size_type

但是有的時後會有問題，例如

std::vector<bool> feature()
{
        return {true};
}

void () {

  bool b1 = feature()[0];
  cout << "b1 type name = " << typeid(b1).name() << endl;
  cout << "b1           = " << b1 << endl;

  auto b2 = feature()[0];
  cout << "b2 type name = " << typeid(b2).name() << endl;
  cout << "b2           = " << b2 << endl;  
}

執行結果

b1 type name = b
b1           = 1
b2 type name = St14_Bit_reference  // std::vector<bool>::reference
b2           = 0

原因是因為std::vector::operator[]回傳的是std::vector::reference，它是一個proxy class。

Example, proxy class

class Proxy{
public:
    operator bool() const { return true; };
    operator int() const { return 123; };
};

void fn()
{
    Proxy p = doSomeThing();

    bool b = doSomeThing();    // b = true
    int i = doSomeThing();     // i = 123
    auto a = doSomeThing()     // a 的 type 是 Proxy
}

Reference

• Effective Modern C++: Item 5: Prefer auto to explicit type declaration
• Effective Modern C++: Item 6: Use the explicitly typed initializer idiom when auto deduces undesired types
• C++ Core Guidelines: Use auto to avoid redundant repetition of type names