在 C++ 中如何评估 if 语句？
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How is if statement evaluated in c++?
提问by derrdji
Is if ( c )the same as if ( c == 0 )in C++?
是if ( c )同if ( c == 0 )在C ++？
回答by Jesper
No, if (c)is the same as if (c != 0).
And if (!c)is the same as if (c == 0).
不，if (c)是一样的if (c != 0)。并且if (!c)与 相同if (c == 0)。
回答by D.Shawley
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.
我会打破这一包......“ if (c)”最接近“ if (((bool)c) == true)”。对于整数类型，这意味着“ if (c != 0)”。正如其他人指出的那样，重载operator !=可能会导致一些奇怪的事情，但operator bool()除非我弄错了，否则重载 " "也会引起一些奇怪的事情。
回答by Patrice Bernassola
If c is a pointer or a numeric value,
如果 c 是指针或数值，
if( c )
is equivalent to
相当于
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())
如果 c 是布尔值（类型 bool [仅 C++]），（编辑：或具有运算符 bool() 重载的用户定义类型）
if( c )
is equivalent to
相当于
if( c == true )
If c is nor a pointer or a numeric value neither a boolean,
如果 c 既不是指针也不是数值，也不是布尔值，
if( c )
will not compile.
不会编译。
回答by Mehrdad Afshari
It's more like if ( c != 0 )
它更像是 if ( c != 0 )
Of course, !=operator can be overloaded so it's not perfectly accurate to say that those are exactly equal.
当然，!=运算符可以重载，因此说它们完全相等并不完全准确。
回答by galets
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:
这仅适用于数值。如果 c 是类，则必须重载一个运算符来进行布尔值转换，例如此处：
#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by Brian R. Bondy
Yes they are the same if you change == 0to != 0.
是的，如果您更改== 0为 ，它们是相同的!= 0。
回答by vikash vishwakarma
if(c)
{
//body
}
The possible value of c are: negative , zero , positive
c 的可能值是：negative , zero , positive
Conditional statement treat * zero* as false
条件语句将 * 零 * 视为 false
While for negative and positive it's true
而对于消极和积极，它是 true
and block will execute only if condition is true.
仅当条件为 时，块才会执行true。
回答by Troubadour
If cis a pointer then the test
如果c是指针，则测试
if ( c )
is not quite the same as
不完全一样
if ( c != 0 )
The latter is a straightforward check of cagainst the 0(null) pointer whereas the former is actually a instruction to check whether cis points to a valid object. Typically compilers produce the same code though.
后者是c对0（空）指针的直接检查，而前者实际上是检查是否c指向有效对象的指令。不过，通常编译器会生成相同的代码。
