How to convert STL string vector into char* in an elegant/quick way?

This splits to smaller stages:

1. How do I calculate the total length of the all the strings in the vector?
2. Allocate the char* mem space
3. 开发者_运维技巧Copy the vector strings into that space

Is there a quick elegant way to do that?

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The most elegant, but perhaps not the quickest, way that I can think of would be the following:

std::string s = std::accumulate(v.begin(), v.end(), std::string());
char * c = new char[s.size() + 1];
std::strcpy(c,s.c_str());

Although I'm not myself a fan of naked dynamic allocation, and personally would never use it, I'll leave that for you to sort out.

If you need to add a space delimiter between the elements, it becomes a bit more complex, and I would probably use a different solution, like this:

std::ostringstream oss;
std::copy(v.begin(), v.end(), std::ostream_iterator<std::string>(oss," "));
size_t sz = oss.str().size();
if(sz) --sz; // truncate trailing space if there is one
char * c = new char[sz+1];
std::copy_n(oss.str().c_str(), sz, c);
c[sz] = 0;

---

Here's a C++0x implementation:

#include <cstddef>
#include <algorithm>
#include <numeric>
#include <string>
#include <vector>

std::vector<char> concat_strings(std::vector<std::string> const& vecs)
{
    std::size_t const cumulative_length = std::accumulate(
        vecs.begin(),
        vecs.end(),
        static_cast<std::size_t>(0),
        [](std::size_t const acc, std::string const& str)
        {
            return acc + str.size();
        }
    );

    std::vector<char> ret;
    ret.reserve(cumulative_length);

    std::for_each(
        vecs.begin(),
        vecs.end(),
        [&ret](std::string const& str)
        {
            ret.insert(ret.end(), str.begin(), str.end());
        }
    );

    return ret;
}

The resulting std::vector<char> is your effective char*.

For C++03 it will be much the same, but you'll need proper functors in place of the lambdas (or use a C++03 lambda library e.g. Boost.Phoenix).

---

It's not an elegant requirement, so there's no elegant solution I can think of.

Untested code

vector<string> vec = ...;
size_t total_size = 0;
for (size_t i = 0; i < vec.size(); ++i)
  total_size += vec[i].size();
char* mem = new char[total_size];
char* p = mem;
for (size_t i = 0; i < vec.size(); ++i)
{
  memcpy(p, vec[i].data(), vec[i].size());
  p += vec[i].size();
}

---

Here is STL way:

#include <algorithm>  // std::copy
#include <iostream>   // std::cout
#include <iterator>   // std::ostream_iterator
#include <sstream>    // std::ostring_stream
#include <string>     // std::string
#include <vector>     // std::Vector

using namespace std;

typedef std::vector<std::string> string_vector;

int main( int argc, char ** argv )
{
  string_vector v;
  v.push_back("string 1, ");
  v.push_back("string 2, ");
  v.push_back("string 3");

  std::ostringstream os;
  std::copy( v.begin(), v.end(), std::ostream_iterator<std::string>( os, "" ) );
  std::string result = os.str();

  std::cout << "result: " << result << std::endl;

  return 0;
}

Output:

result: string 1, string 2, string 3