Looking for a warning when casting 'const char *' to bool.

Looking for a warning when casting 'const char *' to bool.

[Andrew Frezell]

Hello all,

I've done some searching and haven't found the right flag to issue a warning in the following scenario.  When passing a 'const char *' to an overloaded function that takes either a 'char *' or bool, the compiler is casting the 'const char *' to a bool and no warning is issued with the following flags: -Wall -Wextra -pedantic.  I tried a mix of a bunch of other warning flags and no success.  Is there a way for g++ to issue a warning in this case?  Below is example code

I know what the correct thing to do is, provide another overloaded function that takes a 'const char *', but this is an existing code base of millions of lines of code and I would like to find anything lurking through a warning.

Thanks,

Drew

// Start example

#include <iostream>

using namespace std;

class Foo {
public:
    void Test(char *c, bool b);
    void Test(char *c1, char *c2);
};

inline void Foo::Test(char *c, bool b) {
    cout << __PRETTY_FUNCTION__ << endl;
    cout << "c = " << c << endl;
    cout << "b = " << b << endl;
}

inline void Foo::Test(char *c1, char *c2) {
    cout << __PRETTY_FUNCTION__ << endl;
    cout << "c1 = " << c1 << endl;
    cout << "c2 = " << c2 << endl;
}

int main(int argc, char *argv[]) {
    char *c1 = "first arg";
    const char *c2 = "second arg";
    Foo f;

    f.Test(c1, c2);

    return 0;
}


      

Re: Looking for a warning when casting 'const char *' to bool.

[Eljay Love-Jensen]

Hi Drew,

> I've done some searching and haven't found the right flag to issue a warning
> in the following scenario.  When passing a 'const char *' to an overloaded
> function that takes either a 'char *' or bool, the compiler is casting the
> 'const char *' to a bool and no warning is issued with the following flags:
> -Wall -Wextra -pedantic.  I tried a mix of a bunch of other warning flags and
> no success.  Is there a way for g++ to issue a warning in this case?

No, since pointers readily (implicitly) convert to bool -- by design as per
the ISO 14882 standard -- there's no way to avoid that conversion in the
situation you described.

char const* --> bool
char const* !--> char*

Since there is only one selectable routine, there is no ambiguity.  Hence,
no warning (or error).

One workaround I've used is to make my own Bool class that isn't "bool
promiscuous", but that does not appear to be viable / desirable in the
example code you provided.

For your test case, the issue is that the code fails const correctness.

Also, with -Wwrite-strings you should see a warning for this line...
  char* c1 = "first arg";
...since string literals are const.

HTH,
--Eljay

Re: Looking for a warning when casting 'const char *' to bool.

[corey taylor]

On Fri, Aug 22, 2008 at 7:00 AM, Eljay Love-Jensen <eljay@adobe.com> wrote:
> Since there is only one selectable routine, there is no ambiguity.  Hence,
> no warning (or error).
>
> One workaround I've used is to make my own Bool class that isn't "bool
> promiscuous", but that does not appear to be viable / desirable in the
> example code you provided.

Although that work work, I'm not a big fan of redefining base types like that.

Assuming you haven't already have you looked into using std::string
It would provide the functionality and type-safety you want.

It also seems like you have some code quality issues if you have to
generate warnings for incorrect function calls.  Either you should be
able to rely on passing valid parameters to some extent or you need to
fix the parameter types.  Is there a big reason to make the parameter
char* instead of const char*?  Any string literal will be const!

corey