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

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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.
}

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