Quick question on const parameters and functions

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I understand the importance of using const in function declarations but I noticed something that I could use some input on.

The following snippet compiles without error

C++

bool Foo(const char* str)
{
    if ( str = nullptr ) { return false; } //Intentional reassignment
}

However

C++

bool Foo(char* const str)
{
    if ( str = nullptr ) { return false; } //Intentional reassignment
}

Throws an assignment error.

From the eye of compiler, what is the difference between these two function calls?
I guess I am looking for a little clarification on the enigmatic const.

Posted 26-Aug-13 13:42pm

Foothill

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User 59241 · Accepted Answer · 2013-08-26T14:26:00

C++

char* const str

Declares a const pointer. Thus the pointer is immutable and the address value it is initialised with cannot be changed. It can't be pointed to another variable. The variable it points to may be changed.

So

C++

str = nullptr

is not allowed.

But:

C++

const char* str

declares a pointer to a const. Thus the pointer may be changed but the variable it points to may not be.

So

C++

str = nullptr

is allowed.

http://www.thegeekstuff.com/2012/06/c-constant-pointers/[^]

http://www.parashift.com/c++-faq/const-correctness.html[^]