Item 4: Make sure that objects are initialized before they’re used.(2)

The best way to deal with this seemingly indeterminate state of affairs is to always initialize your objects before you use them. For non-member objects of built-in types, you’ll need to do this manually. For example:

int x = 0;  // manual initialization of an int  
const char * text = "A C-style string"; // manual initialization of a  
 // pointer (see also Item 3)  
double d;  // “initialization” by reading from  
std::cin >> d;  // an input stream 

For almost everything else, the responsibility for initialization falls on constructors. The rule there is simple: make sure that all constructors initialize everything in the object.

The rule is easy to follow, but it’s important not to confuse assignment with initialization. Consider a constructor for a class representing entries in an address book:

class PhoneNumber { ... };  
class ABEntry {  // ABEntry = “Address Book Entry”  
public:  
  ABEntry(const std::string& name, const std::string& address,  
             const std::list<PhoneNumber>& phones);  
private:  
  std::string theName;  
  std::string theAddress;  
  std::list<PhoneNumber> thePhones;  
  int numTimesConsulted;  
};  
ABEntry::ABEntry(const std::string& name, const std::string& address,  
                      const std::list<PhoneNumber>& phones)  
{  
  theName = name;  // these are all assignments,  
  theAddress = address;  // not initializations  
  thePhones = phones;  
  numTimesConsulted = 0;  
} 

大部分情况下，我们已经不再需要逐条指令、每个时钟周期地去推敲代码的执行性能。即使你有强迫症，非要这样做，许多现代CPU（尤其是复杂指令集的CPU）的工作方式也不能精确地保证每条指令到底需要多长时间完成。所以，即便你觉得某处变量声明处的初始化过程不必要，也最好养成习惯去初始化变量。

当你把编译器的警告开关打开（以我最常用的编译器gcc为例），编译器通常会检查出那些可能未被初始化就开始使用的变量。你可以强迫编译器在警告的同时停止编译，这么做可以帮助你培养出初始化变量的好习惯。