String::copy error - c++

Question

I have the following code and get the error later described how do i correct this?
The objective behind is to parse the following sentence into variables :
temp1+temp2=10
Code:
int main()
{
string line,var1;
int limit,len;
//some code here
// parse function declarartion :string parse(string ,char ,int &)
f1>>line;
len=line.length();
var1=parse(line,'+',limit);
line.copy(line,len-limit,limit);
//some code here
}
Error:
alice.cpp: In function ‘int main()’:
alice.cpp:65:40: error: no matching function for call to ‘std::basic_string<char>::copy(std::string&, int, int&)’
alicebob.cpp:65:40: note: candidate is:
/usr/include/c++/4.6/bits/basic_string.tcc:724:5: note: std::basic_string<_CharT, _Traits, _Alloc>::size_type std::basic_string<_CharT, _Traits, _Alloc>::copy(_CharT*, std::basic_string<_CharT, _Traits, _Alloc>::size_type, std::basic_string<_CharT, _Traits, _Alloc>::size_type) const [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>, std::basic_string<_CharT, _Traits, _Alloc>::size_type = unsigned int]
/usr/include/c++/4.6/bits/basic_string.tcc:724:5: note: no known conversion for argument 1 from ‘std::string {aka std::basic_string<char>}’ to ‘char*’

Answer 1

If you just want to copy a substring into another string, why don't you use substr() instead?
int main()
{
std::string line("ceva5");
std::string var1;
int limit = 1,len;
//some code here
// parse function declarartion :string parse(string ,char ,int &)
// f1>>line;
len=line.length();
//
// var1=parse(line,'+',limit);
line = line.substr(limit, len-limit);
//some code here
std::cout << line << std::endl;
}
This should do what you want.
EDIT: I have not implemented your function but changed the code to work as string's copy().

Answer 2

Here is the explain: http://www.cplusplus.com/reference/string/string/copy/ ,The first arg is char*, So,Accroding to your question,I think your code should be
line.copy((char *)line.c_str(),len-limit,limit); // wrong code, lost the const
but this code is dangerous,Because std::copy()'s first arg is an array of characters to store the string 's substring, So,I think you code has something wrong.
Here is simple code:
std::string test1 = "test1";
char buffer[10] = {0};
test1.copy(buffer,2,3);
std::cout << "buffer is: " << buffer << std::endl;
and the output is :"buffer is: t1".