invalid conversion from ‘char’ to ‘const char’

Asked Jul 31 '13 at 00:24

Active Jul 31 '13 at 00:24

Viewed 420 times

void foo(const char **p)
{

}

int main()
{
    char *t;
    foo(&t);

    return 0;
}

This code doesn't compile (http://ideone.com/2NHA08) on gcc (error: invalid conversion from ‘char**’ to ‘const char**’ [-fpermissive]), but why? Conversion is an addition of const-ness, why can it fail?

asked Jul 31 '13 at 00:24

hate-engine

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`Conversion is an addition of const-ness, why can it fail?` Your assumption that it cannot is ill-formed. In fact you did not add `const`ness to the object you're passing -- the `char**`, but instead you tried to add `const`ness to the theoretical point-pointee, which is something _completely_ different. Notice that `const char**` is not the same as `char** const`. – Lightness Races in Orbit Jul 31 '13 at 00:28
The *pointer* can be implicitly converted to `const`, but not what it points to. – David G Jul 31 '13 at 00:28
@0x499602D2 Yet, we can convert `char*` to `const char*` just fine. – jrok Jul 31 '13 at 00:29
@jrok: At one level of indirection this can be enforced. At two, it cannot. – Lightness Races in Orbit Jul 31 '13 at 00:31
@LightnessRacesinOrbit Yes. I didn't claim otherwise. – jrok Jul 31 '13 at 00:32
@jrok: And I didn't claim that you did! – Lightness Races in Orbit Jul 31 '13 at 13:44
@LightnessRacesinOrbit Fair enough! – jrok Jul 31 '13 at 13:45
1

It's not clear to me that the C++ answers/solutions are correct for C. – mcr Oct 09 '14 at 23:27

invalid conversion from ‘char’ to ‘const char’

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