How is if statement evaluated in c++?

问题

Is if ( c ) the same as if ( c == 0 ) in C++?

回答1:

No, if (c) is the same as if (c != 0). And if (!c) is the same as if (c == 0).

回答2:

I'll break from the pack on this one... "if (c)" is closest to "if (((bool)c) == true)". For integer types, this means "if (c != 0)". As others have pointed out, overloading operator != can cause some strangeness but so can overloading "operator bool()" unless I am mistaken.

回答3:

If c is a pointer or a numeric value,

if( c )

is equivalent to

if( c != 0 )

If c is a boolean (type bool [only C++]), (edit: or a user-defined type with the overload of the operator bool())

if( c )

is equivalent to

if( c == true )

If c is nor a pointer or a numeric value neither a boolean,

if( c )

will not compile.

回答4:

It's more like if ( c != 0 )

Of course, != operator can be overloaded so it's not perfectly accurate to say that those are exactly equal.

回答5:

This is only true for numeric values. if c is class, there must be an operator overloaded which does conversion boolean, such as in here:

#include <stdio.h>

class c_type
{
public:
    operator bool()
    {
        return true;
    }
};

int main()
{
    c_type c;
    if (c) printf("true");
    if (!c) printf ("false");
}

回答6:

Yes they are the same if you change == 0 to != 0.

回答7:

if(c)
{
  //body
}

The possible value of c are: negative , zero , positive

Conditional statement treat * zero* as false

While for negative and positive it's true

and block will execute only if condition is true.

回答8:

If c is a pointer then the test

if ( c )

is not quite the same as

if ( c != 0 )

The latter is a straightforward check of c against the 0 (null) pointer whereas the former is actually a instruction to check whether c is points to a valid object. Typically compilers produce the same code though.

来源：https://stackoverflow.com/questions/1479100/how-is-if-statement-evaluated-in-c

标签

c++

if-statement

evaluation