C++ char*[] to char** conversion

Question

I have this simple code that compiles without errors/warnings:

void f(int&, char**&){}

int main(int argc, char* argv[])
{
    f(argc, argv);
    return 0;
}

And next similar code that doesn't compile:

void f(int&, char**&){}

int main()
{
    int argc = 2;
    char* argv[] = { "", "", nullptr };
    f(argc, argv); 
    //@VS2013 error: cannot convert argument 2 from 'char *[3]' to 'char **&'
    //@GCC error: invalid initialization of non-const reference of type 'char**&' from an rvalue of type 'char**'
    return 0;
}

Why char*[] can be converted to char**& in the first sample and can't be converted in the second sample? Does it matter if the size is known at compile time?

EDIT: I think there are 2 conversions needed in the second case, and only one implicit conversion can be done by compiler.

This code compiles fine:

void f(int&, char**&){}

int main()
{
    int argc = 2;
    char* temp[] = { "", "", nullptr };
    char** argv = temp;
    f(argc, argv);
    return 0;
}

Answer 1

Because despite appearances, the second argument to main has type char**. When used as the declaration of a function argument, a top level array is rewritten to a pointer, so char *[] is, in fact, char**. This only applies to function parameters, however.

A char*[] (as in your second case) can convert to a char**, but the results of the conversion (as with any conversion) is an rvalue, and cannot be used to initialize a non-const reference. Why do you want the reference? If it is to modify the pointer, modifying the char** argument to main is undefined behavior (formally, in C, at least—I've not checked if C++ is more liberal here). And of course, there's no way you can possibly modify the constant address of an array. And if you don't want to modify it, why use a reference?