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。