CODEWITHSUNDEEP
c++c++11
waz
waz

Reputation: 41

Why a function with Bool argument accepts String?

I have a function which accepts an bool as an argument:

void func(bool a)
{
    doing something;
}

But when I'm calling the function and passed string to it:

func("false");

Actually it should accept only.

func(false);

It accepts string without any error.

why?

Upvotes: 3

Views: 1482

Answers (4)

Kiwi Nick
Kiwi Nick

Reputation: 150

@101010's answer is good. But for the benefit of those with older compilers (which I suspect do not accept that syntax), there is an alternative approach.

First:
A template function declaration with no definition/implementation. That way, if you call it accidentally with func("false") or func('y') or func(0) (an int), the linker will complain. Which will make you look for the problem.

template<typename T> void func(T a);  // prototype only

Declare this alongside your real void func(bool a) function.

If you have this inside a class, you can make the template function/method private, which will make the error into a compiler error (easier to understand, but if the accidental mis-typed call comes from within a class or friend, it will still be a linker error).

Upvotes: 1

Dimitrios Bouzas
Dimitrios Bouzas

Reputation: 42929

According to the C++ standard §4.14/p1 Boolean conversions [conv.bool] (Emphasis Mine):

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. For direct-initialization (8.6), a prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

That is, according the C++ standard, conversion from a string literal (i.e., pointer) to a boolean is a standard conversion and thus what you're experiencing is perfectly normal/standard language behavior.

You can restrict your function to explicitly accept only bool values by defining a generic overload of your function as deleted:

template<typename T> void func(T&&) = delete;

void func(bool a) {
  // do something...
}

Live Demo

Upvotes: 4

Chen OT
Chen OT

Reputation: 3614

Literal string is a char pointer, and pointer will be implicitly converted to bool.

  • Null pointer -> false
  • Non-null pointer -> true

In this case, the func will receive a true value from the non-null "false" string , which is const char* type.

You may need an interpreter function to able to read "false", "0", "no", etc from keyboard typing and convert to bool false which the program can understand.

Upvotes: 5

Stephan Lechner
Stephan Lechner

Reputation: 35154

Pointers can be implicitly converted to bool; The value is false, if the pointer is NULL, and it is true if the pointer is !NULL. Hence, since you pass a pointer to char *-literal "false", this pointer is ! NULL and yields true. See the following definition in the cpp reference concerning of implicit conversion:

Boolean conversions

A prvalue of integral, floating-point, unscoped enumeration, pointer, and pointer-to-member types can be converted to a prvalue of type bool. The value zero (for integral, floating-point, and unscoped enumeration) and the null pointer and the null pointer-to-member values become false. All other values become true

Upvotes: 2

Related Questions