68

I used the strcpy() function and it only works if I use C-string arrays like:

char a[6] = "text";
char b[6] = "image";
strcpy(a,b);

but whenever I use

string a = "text";
string b = "image";
strcpy(a,b);

I get this error:

functions.cpp: no matching function for call to strcpy(std::string&, std::string&)

How to copy 2 strings of string data type in C++?

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2
  • 17
    Just a=b; :D ____ Commented Oct 1, 2012 at 18:24
  • You should use the std::string type and not use strcpy but copy assignment. Commented Oct 1, 2012 at 18:24

4 Answers 4

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97

You shouldn't use strcpy() to copy a std::string, only use it for C-Style strings.

If you want to copy a to b then just use the = operator.

string a = "text";
string b = "image";
b = a;
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6 Comments

and what of strcat () ? is it a=a+"image"; ?
for strcat you can use a+="image"
All of strcpy, strcat, strtok etc. are functions of string.h which operator only on c strings i.e. strings that are mere character arrays terminated by a null. If for convenience you have a string object and you still want to use some string.h function, you can call str.c_str() on the string object which basically returns a c string for the contents of the object. However, this is seldom needed because string object has a pretty good api
@Caesar, please do answer this : link; Thanks.
If for example I would wish for a copy of string that was "on the stack" (ie. say passed by function call) to be put into a struct that lives on the heap, will this still hold?
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22

strcpy is only for C strings. For std::string you copy it like any C++ object.

std::string a = "text";
std::string b = a; // copy a into b

If you want to concatenate strings you can use the + operator:

std::string a = "text";
std::string b = "image";
a = a + b; // or a += b;

You can even do many at once:

std::string c = a + " " + b + "hello";

Although "hello" + " world" doesn't work as you might expect. You need an explicit std::string to be in there: std::string("Hello") + "world"

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7 Comments

and what is the alternative of strcat for std::string ?
@Programer You can combine strings using +: std::string a = "text"; std::string b = "image"; a = a+b;
@PiotrNycz He asks for copying without specifying copy assignment or copy construction, probably because that distinction doesn't make sense for C strings. New C++ programmers typically don't understand the difference between initialization and assignment anyway, and the syntax is deliberately similar. I don't see any reason for the distinction to matter here.
@bames53 OP has two string objects of two distinct values, then wants to overwrite one sting by value of other, This is assignment. But this is your answer not mine. I just thought you overlooked this.
@mLstudent33 No, the std::string copy is entirely independent.
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4

strcpy example:

#include <stdio.h>
#include <string.h>

int main ()
{
  char str1[]="Sample string" ;
  char str2[40] ;
  strcpy (str2,str1) ;
  printf ("str1: %s\n",str1) ;
  return 0 ;
}

Output: str1: Sample string

Your case:

A simple = operator should do the job.

string str1="Sample string" ;
string str2 = str1 ;
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1 Comment

does that mean it's not possible to cast from one type string to other type?
4

Caesar's solution is the best in my opinion, but if you still insist to use the strcpy function, then after you have your strings ready:

string a = "text";
string b = "image";

You can try either:

strcpy(a.data(), b.data());

or

strcpy(a.c_str(), b.c_str());

Just call either the data() or c_str() member functions of the std::string class, to get the char* pointer of the string object.

The strcpy() function doesn't have overload to accept two std::string objects as parameters. It has only one overload to accept two char* pointers as parameters.

Both data and c_str return what does strcpy() want exactly.

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3 Comments

" It has only one overload". Is this really true? Isn't it the case that it's no overloads? Or that's how I understood the concept of overload.
Should you have the urge to use this, take a cold shower and just don't. The size of the underlying buffer is only guaranteed to be the size of the c-string. Fitting "image" into "text" might work without causing horrible things to happen, but WILL result in the string "imag", since the length of std::string is stored and not evaluated by the terminating character. Plus the second variant will probably not even compile, because c_str() returns a const char *.
Don't ever do this; buffer overflow is very easy to trigger with this. Downvoted.

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