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c++pointers
J. Doe
J. Doe

Reputation: 915

What is the difference in pointers when defining char and int?

I understand the basics of how pointers work, but the following example confuses me.

int *myNum = 10; // Produces an error

char *myChar = "Something"; // Works fine

Why does assigning char work but integer doesn't (Maybe cause char is treated as an array)?

As well what confuses me when directly assigning a pointer variable, does it automatically get an address?

char *myChar = "Something";

and

char myChar = "Something";
char *charAddr = &myChar;

What would be the difference here, or equals?

Upvotes: 14

Views: 3012

Answers (5)

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Criticize SE actions means ban

Reputation: 58858

"Something"

is essentially short for:

static const char some_hidden_array[] = {'S', 'o', 'm', 'e', 't', 'h', 'i', 'n', 'g', '\0'};
some_hidden_array

That is, when you write "Something", the compiler generates an array behind the scenes, and gives you a pointer to the start of that array. Since this is already a pointer to a char, you'll have no problem assigning it to a variable of type "pointer to a char" (written as char*).

10

is not short for anything similar. It's just the number 10 - it's not a pointer to an array containing the number 10, or anything like that.

Note that a char is a single character, not a string, which is why the string syntax is unusual compared to most other types - a string is several chars, not just one. If you try to use a plain old char, you'll see the same thing:

char *myChar = 'a'; // error

or for any other type:

float *myFloat = 42.1f; // error

In other words, it's not strange that 10 gives an error - if anything, it's strange that "Something" doesn't. (At least, it's strange until you know how string literals work)

Upvotes: 22

Ivan Gritsenko
Ivan Gritsenko

Reputation: 4236

Why does assigning char work but integer doesn't (Maybe cause char is treated as an array)?

You are right, "Something" is a string literal and can be treated as char array. After char *myChar = "Something"; the following thing happen: it is allocated length+1 bytes of memory where "Something" will be stored, myChar is pointed to the starting address of this memory. String literals are somewhat special.

Here is a general way of initializing array with constant values:

// valid initializations;
char s2[] = { 'a', 'b', 'c' };
int a[] = { 1, 2, 3 };
char s1[] = "123";

As well what confuses me when directly assigning a pointer variable, does it automatically get an address?

Yes.

Take a look at 8.5.2 Character arrays of c++ docs

Upvotes: 3

Sahil Arora
Sahil Arora

Reputation: 885

When you do char *myChar = "Something";, you create a read-only string literal somewhere in the memory, which ends in a null character. Now this is something special with the compiler, that it interprets a chunk of 'char' variables stored continuously and ending with a null character as string. So basically you created an array of characters, and when you do *myChar*, it returns the string.

In case of integers or any other data types, it differentiates between int *ptr as a pointer to an integer, and int ptr as an integer. You are getting an error probably because the address you entered may not be valid/available to you.

Also, doing 

char myChar = "Something";  //this is an error, since char can hold one character
char *charAddr = &myChar;

Note that myChar and &myChar are the same, since myChar is a pointer!

Edit: Refer here about string literals: Is it possible to modify a string of char in C?

Upvotes: 2

wallyk
wallyk

Reputation: 57774

While theoretically the first int *myNum = 10 makes sense—especially if you know there is a useful int at address ten—in general it is rarely useful and potentially quite dangerous.

However, there are certain pointer assignments which are widely used and quite safe:

int *myNum = 0;

On 99.9+% of modern CPU architectures, this is the same as

int *myNum = NULL;

See the definition of NULL in <stddef.h> here.

As a general rule, assignment of pointer variables is best done by setting to the address of something else.

int k, *p = &k;

Upvotes: 1

John Difool
John Difool

Reputation: 5702

It's the same thing (no magic from the compiler is happening). By default, literals like 10 are int values, not int*.

You need to cast:

int *myNum = (int*)10; // Need to cast
char *myChar = "Something"; // No need to cast "..." is already a char*

Note that it's dangerous to reference a pointer to absolute value like this because you will end up with the address 10 in CPU memory.

Regarding your second question, "..." is treated as a contiguous sequence of char in memory similar to array and equivalent to char*.

For a thoughtful understanding of C, pointers and differences between arrays and pointers, you should read this: Expert C Programming: Deep C Secrets by Peter van der Linden.

Upvotes: 10

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