Passing char* to a function and change the argument problem

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How can I call 'ChangeString' from main and have the function change the passed char pointer to some other string value? Thanks in advance.

#include using namespace std;

void ChangeChar(char* value)
{
*value = 'b';
}

void ChangeString(char* value[])
{
*value = "Changed!";
}

int main()
{
char c = 'a';
cout << c << endl;
ChangeChar(&c); // OK!
cout << c << endl;

char str\[\] = "Hello World";
cout << str << endl;
// ChangeString(&str); << error!
cout << str << endl;

system("pause");
return 0;

}

CPallini

Technically you need a double pointer (a pointer to a pointer to char), you may use a reference:

void ChangeString(char* & value)
{
value = "Changed!";
}

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
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[My articles]

Lost User

In the following line str is an array, not a pointer so it cannot be changed by the ChangeString() function.

char str[] = "Hello World";

If you change it to

char* str = "Hello World";

then it should work.

The best things in life are not things.

_Superman_

The other answers tell you how it can be done. But there is a risk factor here, in that the size of the buffer is not known to the ChangeString function. So it is always safe to pass in a second size parameter and only copy that many characters to the buffer inside the ChangeString function.

«_Superman_» _I love work. It gives me something to do between weekends.

_Microsoft MVP (Visual C++)

Polymorphism in C

Lost User

The issue is about changing a pointer whose address is given, not copying into a buffer, so no length is necessary.

The best things in life are not things.

MicroVirus

Actually this is bad practice. This code is good:

char str[] = "Hello World";

This code is bad:

char* str = "Hello World";

The difference? In the first example str is allocated on the stack and you have full read and write access to it. On the second, str points to a memory location containing the string "Hello World", but this memory could be allocated in a static (read only) section. Or consider a piece of code where you have this string literal multiple times. A compiler might decide to store it once and make all references to the string point to the same location. Then when you change it in one location it changes in all, but that wasn't your intention! Instead, you should consider using a const char*, or if you want to modify it use string functions.

const char* str = "Hello World";
// Or:
char str[200] = "Hello World";
//now you can place strings up to length 199 (+null-termination) in str using memcpy's or string functions

Lost User

Without knowing the intention of the programmer, neither of your suggestions is 'good' or 'bad'. Either way, it has little to do with the OP's question.

The best things in life are not things.

MicroVirus

Unfortunately, it has everything to do with your answer and code. Even though the OP is obviously new to C/C++, it doesn't hurt to learn good practices right from the start. At any rate, it wasn't a suggestion: it's me saying that it is bad practice to use pointers and string literals in that way, for the reasons I explained. This is quite independent of the programmer's intentions. So my point is: your solution was almost correct, except for that you should use 'const char*' everywhere to work with it like this. This could potentially save the OP lots of time trying to debug weird access violation errors later on, as the compiler will be able to inform you when you are doing something inappropriate that you are allowed to do when just defining it as 'char*'. Now if you think I am entirely wrong and that it is in fact not poor practice, then I'd love to hear your arguments.