编译器允许'char * k = false'？为什么？

Question

I found strange from my point of view compiler behavior, It allows assign Boolean value to * char .

char * k= false;

Why? But after assignment * char is still not initialized. Why compilers doesn't allows assign int value?

Answer 1

It will be implicitly converting the boolean value false to an integer with value zero and as such declaring a NULL pointer. It is effectively no different from

char* k = 0;

which is valid syntax

Answer 2

C++03 Standard, #4.10:

A null pointer constant is an integral constant expression (5.19) rvalue of integer type that evaluates to zero.

5.19:

An integral constant-expression can involve only literals (2.13), enumerators, const variables or static data members of integral or enumeration types initialized with constant expressions (8.5), non-type tem- plate parameters of integral or enumeration types, and sizeof expressions.

false is a boolean literal, therefore it falls into the category of a constant expression, so it can qualify as a null pointer constant.

Answer 3

false and true are shortcuts for 0 and 1 . For pointer you use NULL which define NULL 0 . So it correct syntax.

Answer 4

The compiler allows it because in C++ false is the same as 0 and NULL .

Personally, at least for assignments, I find it easier to understand and more correct to use NULL to indicate a null pointer.

Btw, before C++, on some systems NULL was actually a macro defined as (void*)0xffff ; some background on that can be found in this answer .