如何转换矢量 <string> 到一个向量 <char*>

Question

This is the opposite of How to convert a vector<char*> to a vector<string>/string question.

I have some legacy routine which works with vector<char*> so I need to transform my vector<string> .

Here is what I come out with:

std::vector<char*> charVec(strVec.size(),nullptr);
for (int i=0; i<strVec.size();i++) {
    charVec[i]= new char(strVec[i].size()+1); 
    charVec[i][strVec[i].copy(charVec[i], strVec[i].size())] = '\0';
}

Is this correct?

Is there a better way to implement it?

---

ps of course at the end I have:

for (int i=0; i<strVec.size();i++) {
    delete charVec[i];
}

Answer 1

A faster way of doing this is

std::vector<const char*> charVec(strVec.size(),nullptr);
for (int i=0; i<strVec.size();i++) {
    charVec[i]= strVec[i].c_str();
}

and then using the resulting vector. This will save a lot of time on memory allocation for large sets of data.

Answer 2

Why are you allocating new buffer for your strings? You are passing a vector of character pointers to a function and finally deleting allocated buffers. It means these buffers are temporary and there is no need to allocate memory for them as far as source vector ( vector<string> ) exist.

You can easily do this:

std::vector<std::string> vec1 = { "1", "2", "3" };
std::vector<const char*> vec2;

vec2.resize(vec1.size(), nullptr);

std::transform(std::begin(vec1), std::end(vec1), std::begin(vec2), [&](const std::string& str)
{
    return str.c_str();
});

Answer 3

Your new statement looks wrong. You want to allocate an array of char long enough to hold the string. You're allocating a single char whose value is the length of the string. I'm surprised you didn't get compiler warnings about that.

Try this:

std::vector<char*> charVec;
for (const auto &str : strVec) {
  char *charStr = new char[str.size() + 1];
  std::strcpy(charStr, str.c_str());
  charVec.push_back(charStr); 
}

The only drawback here is that, if one of the strings has embedded null characters, nothing beyond those will be copied. But I suspect that a function which takes a vector of char * almost certainly doesn't care about that anyway.

Answer 4

You could simply use string::c_str() to get the c string out of a std::string. Read more in How to convert a std::string to const char* or char*?

Or you could just modify a bit the body of the function, but only you know if this is worth it.

---

Of course as ssell mentions in the comments, keep in mind that with this approach you need not to destroy your strings when you handle them! It's a trade-off, you avoid the copy (so cool), but you need to be careful not reading junk (in case you delete your strings)!

Answer 5

vector < string > to vector < const char* > : use c_str()

std::vector<const char*> charVec(strVec.size(), nullptr);
for (int i=0; i<strVec.size(); i++) {
    charVec[i] = strVec[i].c_str();
}

vector < string > to vector < char* > : use &x[0]

std::vector<char*> charVec(strVec.size(), nullptr);
for (int i=0; i<strVec.size(); i++) {
    charVec[i] = &strVec[i][0];
}

Refs: How to convert a std::string to const char* or char*?