Why is a C++ bool var true by default?

前端 未结 7 1667
野趣味
野趣味 2020-12-05 18:51

bool \"bar\" is by default true, but it should be false, it can not be initiliazied in the constructor. is there a way to init it as false without making it static?

相关标签:
7条回答
  • 2020-12-05 19:39

    You can initialize any variable when you declare it.

    bool bar = 0;
    
    0 讨论(0)
  • 2020-12-05 19:43

    This program has undefined behavior. Intrinsic types do not have constructors. You could do bool bar = bool(); but it's better to define the actual value in your foo class.

    0 讨论(0)
  • 2020-12-05 19:45

    Try this:

    class Foo
    { 
      public: 
        void Foo(); 
      private: 
         bool bar; 
    } 
    
    Foo::Foo() : bar(false)
    {
      if(bar) 
      { 
        doSomething(); 
      }
    }
    
    0 讨论(0)
  • 2020-12-05 19:50

    Since you're using the value of bar in the ctor, I assume you're trying to set it before the object is created? I think you meant to make it a static class variable instead of an instance variable.

    0 讨论(0)
  • 2020-12-05 19:51

    In fact, by default it's not initialized at all. The value you see is simply some trash values in the memory that have been used for allocation.

    If you want to set a default value, you'll have to ask for it in the constructor :

    class Foo{
     public:
         Foo() : bar() {} // default bool value == false 
         // OR to be clear:
         Foo() : bar( false ) {} 
    
         void foo();
    private:
         bool bar;
    }
    

    UPDATE C++11:

    If you can use a C++11 compiler, you can now default construct instead (most of the time):

    class Foo{
     public:
         // The constructor will be generated automatically, except if you need to write it yourself.
         void foo();
    private:
         bool bar = false; // Always false by default at construction, except if you change it manually in a constructor's initializer list.
    }
    
    0 讨论(0)
  • 2020-12-05 19:52

    Klaim's answer is spot on. To "solve" your problem you could use a constructor initialization list. I strongly suggest you read that page as it may clear up some similar queries you may have in future.

    0 讨论(0)
自定义标题
段落格式
字体
字号
代码语言
提交回复
热议问题