C++的C4305和C4800的编译警告

Contents

今天在改代码时发现了这两个warning，C4305 - ‘identifier’ : truncation from ‘type1’ to ‘type2’ 和C4800 - ‘type’ : forcing value to bool ‘true’ or ‘false’ (performance warning)。

大家看看下面的代码吧，一目了然。：）

int a1 = 1;
int a2 = 2;
MyEnum m = MyEnum::a;

bool b = 1;             // no warning
b = 2;                  // warning C4305: '=' : truncation from 'int' to 'bool'
b = a1;                 // warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning)    
b = a2;                 // warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning)    
b = MyEnum::a;          // no warning
b = m;                  // warning C4800: 'MyEnum' : forcing value to bool 'true' or 'false' (performance warning)    
b = !!a2;               // no warning
b = a2!=0;              // no warning