# Strings copyingmore than expected

by taupune
Tags: copyingmore, expected, strings
 P: 25 Hi ,I am doing some strings operation but it seems that compiler is doing extra copping. Can someone else verify this or is it my sloppy coding? Also I had to write std::cin .....etc otherwise compiler would like the code without that expression in main function. I am using Visual studio 2010. #include using namespace std; #include "string.h" void disect (char TheMessage[], int startloc, char messagepart[], int messagelength); void ResultCodeTable ( char Result[]); int main () { char test[3]; char message [140]; char enter; std::cout << "hello enter the mesage: "<> message; std:: cout<<"uncopied string: " <> enter; return (0); } //Dissect cuts the mesage into several parts , where each part provides the separate meaning of itself void disect (char TheMessage[], int startloc, char messagepart[], int messagelength) { cout<< "this is pre message part inside function printing: "<
P: 4,572
 Quote by taupune Hi ,I am doing some strings operation but it seems that compiler is doing extra copping. Can someone else verify this or is it my sloppy coding? Also I had to write std::cin .....etc otherwise compiler would like the code without that expression in main function. I am using Visual studio 2010. #include using namespace std; #include "string.h" void disect (char TheMessage[], int startloc, char messagepart[], int messagelength); void ResultCodeTable ( char Result[]); int main () { char test[3]; char message [140]; char enter; std::cout << "hello enter the mesage: "<> message; std:: cout<<"uncopied string: " <> enter; return (0); } //Dissect cuts the mesage into several parts , where each part provides the separate meaning of itself void disect (char TheMessage[], int startloc, char messagepart[], int messagelength) { cout<< "this is pre message part inside function printing: "<
This is just a dummy post to put code tags around your code. As a pointer if you post code in the future but CODE and /CODE tages with square braces around them.
P: 77
 Quote by taupune Hi ,I am doing some strings operation but it seems that compiler is doing extra copping. Can someone else verify this or is it my sloppy coding?

void disect (char TheMessage[], int startloc, char messagepart[], int messagelen
gth)
{
cout<< "this is pre message part inside function printing: "<<messagepart<<e
ndl;
cout<<"start loc is:"<<startloc<<" end loc is "<<messagelength<<endl;

int i;
for ( i = 0; i<messagelength; i++)
{
messagepart[i]= TheMessage[startloc];
startloc++;

cout<<"message part array : "<<messagepart[i]<<endl;
cout<<"message original mesage : "<<TheMessage[i]<<endl;
}
messagepart[i] = 0;

cout<< "end of function execution inside function printing: "<<messagepart<<
endl;
}
A lot of standard library functions expect a 0 ('\0') to indicate end of string.

Also if you use C++ strings instead of C strings, the program becomes much easier
#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::string;

int main()
{
string message;
string test;

cout<<"Enter message: ";
cin>>message;

test = message.substr(3,3);
cout<<test<<'\n';

}

P: 77
 Quote by chiro Ok I had a look at your code and I will share my thoughts. The first thing is that it seems that are you passing the two arguments for your disect function by value and not by reference. The difference between the two is that passing by value passes a copy of the actual data on the stack to the function whereas passing by reference gives the function a pointer to the object. In C, you use a pointer to the object to pass by reference and if you are using C++ you can use an ampersand '&' to pass the pointer to the function, but still treat the object like you are treating it in your function. If you are unaware of these things it would be helpful for your to look these up.
His argument passing has no problems.

char message[140];

to function f, if you pass
f(message);

What you are actually passing is
f(&message[0]);

So you are passing a pointer to the first element of the array.
So a copy is created, but not a copy of the array, but a copy of the pointer, which is exactly what you want.

 Quote by chiro Also you have not initialized your character arrays and you are printing the test array that is not initialized at the beginning of your code.
message gets initialized by cin
test gets initialized in disect

 Quote by chiro You also need to make sure the strings are terminated properly. I don't know if this was a mistake or not.
message gets terminated by 0 ('\0') by cin.
test is not - that's the main mistake in his code.
P: 4,572
 Quote by phiby His argument passing has no problems. char message[140]; to function f, if you pass f(message); What you are actually passing is f(&message[0]); So you are passing a pointer to the first element of the array. So a copy is created, but not a copy of the array, but a copy of the pointer, which is exactly what you want.
I think I've mixed up something here.

If you pass say something like char myString[140], then that gets put on the stack but if you pass it without a size then that gets passed by reference. It's been a while since I've done any serious coding so thanks for the correction.
Mentor
P: 21,216
 Quote by chiro I think I've mixed up something here. If you pass say something like char myString[140], then that gets put on the stack
No. What gets passed is the address of the string in memory, which is to say &myString[0], just as phiby said.

This is the way it works in C and C++ if you're working with strings as arrays of type char.

In general, the name of an array is an address, so an actual parameter that is the name of an array is passed by reference.
 Quote by chiro but if you pass it without a size then that gets passed by reference. It's been a while since I've done any serious coding so thanks for the correction.
P: 4,572
 Quote by Mark44 No. What gets passed is the address of the string in memory, which is to say &myString[0], just as phiby said. This is the way it works in C and C++ if you're working with strings as arrays of type char. In general, the name of an array is an address, so an actual parameter that is the name of an array is passed by reference.
So just do I don't forget next time, how do you pass an array on the stack in C/C++ functions? As an example, a character array?
P: 77
 Quote by chiro So just do I don't forget next time, how do you pass an array on the stack in C/C++ functions? As an example, a character array?
There is no simple way to pass a copy of the whole array in C (you could do it in a twisted way - by wrapping the array in a struct & passing the struct by value).

In C++, if you have a std::string, it can be passed by value semantics.

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