string - char* mix and match??

[nigelhoath]

After 3 months of 'C++' (previous languages did not include 'C') I think it's great. But I still find the 'C' bits and the 'C++' bits trip me up.

A simple example is that I generate a format string, as a string type, only to discover printf rejects the string and wants a char*.

My questions:

Is the above true or am I just missing some simple conversion?
Should I try to mix them (C and C++) to the minimum or is the marriage simply the way it is?

String types just seem natural and char* so alien.

Cheers Nigel

[ZuK]

only to discover printf rejects the string and wants a char*.

actually printf wants a const char * for its format string.
That conversion is simple

std::string fmt("%s");
printf(fmt.c_str(), "something" );

Kurt

[Speedo]

A simple example is that I generate a format string, as a string type, only to discover printf rejects the string and wants a char*.

Really, you should try as much as possible to avoid using the printf family of functions in C++ anyway. They aren't typesafe and can only be used with built-in types. As much as possible you should prefer to use stringstreams or, for cleaner more printf like syntax, use boost::format.

[Muthuveerappan]

I am a newbie to C++, I have summed up my understanding (correct me if I am wrong)

*** printf is mainly used in C, so its best to avoid using printf


Its good to to know 2 basic things:


1) char is a built in data type - It stores as an array of characters terminated by '\0'. So always treat it like an character array.

2) string - It is a class defined in the STL library, so treat it like a class, and it has many operators overloaded for convenience.


I have summed up my understanding of char and strings:

char
-----
- char is a built in data type
- char array is capable of storing a string, each element of the char array contains one character
- The end of the string is marked by the null character '\0'
- The size of the char array has to be one bigger than the length of the string it is going to store
- Like any other array, it starts with the index 0, and other properties of an array are applicable (such as can't be compared / assigned directly, will have to be compared / assigned element by element of the array or can use the char functions such as strcmp or strcpy)
- Remember to use '' when you assign values to every element of the char array.
- An example:

case1:
-------
char a[6] = "Hello"; //This is valid, because this is initialization

case2:
-------
char a[6] = { 'H', 'e', 'l', 'l', 'o', '\0'}; // This is the same as above

case3:
--------
char a[6];
a = "Hello"; //This is invalid, because this is an assignment and not initialization, and when you want to assign you have to go element by element or use strcpy

a[0]='H'; //valid


String
--------
- Always remember this is a class and treat accordingly.
- This has many operators overloaded for convenience
- Assignment is legal
- when you declare an object (variable) against this class, the object is capable of storing a string of any size, you don't have to worry about the size
- It also has a set of functions that can be invoked.

string a;
a = "Hello"; //Assignment is valid

cout << a.length(); //objectName.functionName()

[STLDude]

If you want to use C++, don't use printf, use C++ idioms.
boost::format, boost::lexical_cast, and tried and true stringstream (here is sample of stringstream)

C does not offer type safety, where C++ (the one I mentioned) have type safety. If you don't grok that, you will in the future.

This http://www.codeguru.com/forum/showth...35#post1807435 post has a little on boost if you are not familiar.
Also, std::string do provide const char *, you use std::string::c_str() member.

[nigelhoath]

Many thanks STLdude. I'll chase those down. Never heard of boost so I guess that will get me back on strings track.

Yup I'd like to stick to C++. I find it better after each prog.

[Marc G]

[ merged threads ]

Please do not start multiple threads on the same issue.

[potatoCode]

After 3 months of 'C++' (previous languages did not include 'C') I think it's great. But I still find the 'C' bits and the 'C++' bits trip me up.

A simple example is that I generate a format string, as a string type, only to discover printf rejects the string and wants a char*.

That's kind of odd.
If you had no previous knowledge of C, and going straight to C++,
at what point did you come across the printf?

[potatoCode]

Hello Muthuveerappan

Its good to to know 2 basic things:


1) char is a built in data type - It stores as an array of characters terminated by '\0'. So always treat it like an character array.

I think it's good to clarify that char itself is not an array. It can only hold a single char. Not all char arrays are null terminated. I think what you're refering to is the C-style character strings (null terminated char array). Another good thing to keep in mind is that string literal is an array of const char.