C++ : Meaning of const char*const*

Question

In one of the C++ programs, I saw a function prototype: int Classifier::command(int argc, const char*const* argv)

What does const char*const* argv mean? Is it the same as const char* argv[] ? Does const char** argv also mean the same?

Answer 1

From the C++ Super-FAQ :

Read the pointer declarations right-to-left.

• const X* p means " p points to an X that is const ": the X object can't be changed via p .
• X* const p means " p is a const pointer to an X that is non-const ": you can't change the pointer p itself, but you can change the X object via p .
• const X* const p means "p is a const pointer to an X that is const ": you can't change the pointer p itself, nor can you change the X object via p .

And, oh yea, did I mention to read your pointer declarations right-to-left?

const char * const * is the same as char const * const * : a (non-const) pointer to a const pointer to a const char.

const char * is the same as char const * : a (non-const) pointer to a const char.

const char * * is the same as char const * * : a (non-const) pointer to a (non-const) pointer to a const char.

Answer 2

No, it's not the same as const char *argv[] . The const prohibits modifications of the dereferenced value at the particular level of dereferencing:

**argv = x; // not allowed because of the first const
*argv = y; // not allowed because of the second const
argv = z; // allowed because no const appears right next to the argv identifier

Answer 3

const char*const* argv means "pointer to constant pointer to constant char ". It's not "the same" as const char *argv[] , but it is compatible to some extent:

void foo(const char *const *argv);

void bar(const char **argv)
{
    foo(argv);
}

compiles just fine. (The reverse wouldn't compile without a const_cast .)

Answer 4

A pointer that does not change to a string that does not change:

const char* aString ="testString";

aString[0] = 'x';   // invaliv since the content is const
aString = "anotherTestString"; //ok, since th content doesn't change

const char const* bString = "testString";
bString [0] = 'x'; still invalid
bString = "yet another string"; // now invalid since the pointer now too is const and may not be changed.

Answer 5

const char*const* a;

basically says: a is a pointer to an unchangeable const char pointer.

so this would be a valid code: const const const char*const*const* a;