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

问题

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?

Simplified version of the code:

foo.h

class Foo{
 public:
     void Foo();
private:
     bool bar;
}

foo.c

Foo::Foo()
{  
   if(bar)
   {
     doSomethink();
   }
}

回答1:

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.
}

回答2:

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.

回答3:

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.

回答4:

C / C++ don't initialize variables for you at all. The memory location which is now in use by bar had a value in it which is interpreted as "true". This will not always be the case. You must initialize it in your constructor.

回答5:

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.

回答6:

Try this:

class Foo
{ 
  public: 
    void Foo(); 
  private: 
     bool bar; 
} 

Foo::Foo() : bar(false)
{
  if(bar) 
  { 
    doSomething(); 
  }
}

回答7:

You can initialize any variable when you declare it.

bool bar = 0;

来源：https://stackoverflow.com/questions/2043823/why-is-a-c-bool-var-true-by-default

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c++

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boolean

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