I have a std::vector<std::string>
that I need to use for a C
function's argument that reads char* foo
. I have seen how to convert a std::string
to char*
. As a newcomer to C++
, I'm trying to piece together how to perform this conversion on each element of the vector and produce the char*
array.
I've seen several closely related SO questions, but most appear to illustrate ways to go the other direction and create std::vector<std::string>
.
You can use std::transform
as:
std::transform(vs.begin(), vs.end(), std::back_inserter(vc), convert);
Which requires you to implement convert()
as:
char *convert(const std::string & s)
{
char *pc = new char[s.size()+1];
std::strcpy(pc, s.c_str());
return pc;
}
Test code:
int main() {
std::vector<std::string> vs;
vs.push_back("std::string");
vs.push_back("std::vector<std::string>");
vs.push_back("char*");
vs.push_back("std::vector<char*>");
std::vector<char*> vc;
std::transform(vs.begin(), vs.end(), std::back_inserter(vc), convert);
for ( size_t i = 0 ; i < vc.size() ; i++ )
std::cout << vc[i] << std::endl;
for ( size_t i = 0 ; i < vc.size() ; i++ )
delete [] vc[i];
}
Output:
std::string
std::vector<std::string>
char*
std::vector<char*>
Online demo : http://ideone.com/U6QZ5
You can use &vc[0]
wherever you need char**
.
Note that since we're using new
to allocate memory for each std::string
(in convert
function), we've to deallocate the memory at the end. This gives you flexibility to change the vector vs
; you can push_back
more strings to it, delete the existing one from vs
, and vc
(i.e vector<char*>
will still be valid!
But if you don't want this flexibility, then you can use this convert
function:
const char *convert(const std::string & s)
{
return s.c_str();
}
And you've to change std::vector<char*>
to std::vector<const char*>
.
Now after the transformation, if you change vs
by inserting new strings, or by deleting the old ones from it, then all the char*
in vc
might become invalid. That is one important point. Another important point is that, you don't need to use delete vc[i]
in your code anymore.