Why is it possible to pass a string as a char pointer

Question

I have a part of code, I don't understand how it works.

I have int Save(int _key, char *file); And this method Save accepts string as a char pointer Save(i, "log.txt");

So what happens at the end is inside the Save method I use fopen(file, "a+") and it works perfectly fine.

However I don't understand how it accepts "log.txt" for char *file.

Can you be clearer about what you don't understand? Why do you think it shouldn't be accepted? Is the question about strings versus pointers or is it about const pointers versus non-const pointers? — David Schwartz, Oct 07 '15 at 14:54

score 8 · Answer 1 · edited May 23 '17 at 12:23

8

The string literal "log.txt" has type char const[N], as per §2.13.5/8:

Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals. A narrow string literal has type “array of n const char”, where n is the size of the string as defined below, and has static storage duration (3.7).

which decays to a pointer when passed as argument, as per §4.2/1:

An lvalue or rvalue of type “array of N T” or “array of unknown bound of T” can be converted to a prvalue of type “pointer to T”. The result is a pointer to the first element of the array.

The reason char const* can be assigned to char* is mostly there for backward compatibility reasons and it has been deprecated in C++11.

edited May 23 '17 at 12:23

Community

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answered Oct 07 '15 at 14:39

Shoe

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is it `char const[N]` or `const char[N]`? can't remember if there's a difference for arrays(but I believe there is for pointers) – Alex Díaz Oct 07 '15 at 14:41
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@AlejandroDíaz There's no difference. The difference if for when you have `const` before or after `*`, in which case it refers to the constness of the value or the pointer respectively. – Shoe Oct 07 '15 at 14:42

Alex Díaz · Answer 2 · 2015-10-07T14:58:06.607

5

"log.txt" isn't a std:string is actually an array of chars containing {'l','o','g','.','t','x','t','\0'}, its type is const char[N] which decays to const char* hence the call to Save(i, "log.txt"); works.

The call works but the compiler prints a warning stating that converting from const char* to char* has been deprecated in C++03 and invalid in C++11.

edited Oct 07 '15 at 14:58

answered Oct 07 '15 at 14:35

Alex Díaz

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Isn't his question why `const char *` accepted when function expects `char *` ? – Slava Oct 07 '15 at 14:45
@Slava It might be. I can't tell. – David Schwartz Oct 07 '15 at 14:54

score 0 · Answer 3 · answered Oct 07 '15 at 16:34

0

In C++ it is perfectly fine to initialize a character pointer with string literal. After initialization we can use that character pointer like an array as in:

{
  char *s="abc";
  cout<<s[0];
  cout<<s[1];
}

answered Oct 07 '15 at 16:34

user3297764

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Why is it possible to pass a string as a char pointer

3 Answers3