CODEWITHSUNDEEP
c++stlcharstdvector
bibi
bibi

Reputation: 3765

How to convert a vector<string> to a vector<char*>

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?

p.s. of course at the end I have:

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

Upvotes: 4

Views: 6711

Answers (5)

mdrafiqulrabin
mdrafiqulrabin

Reputation: 1697

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*?

Upvotes: 0

gsamaras
gsamaras

Reputation: 73366

You could simply use string::c_str() to get the 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)!

Upvotes: 2

JeremiahB
JeremiahB

Reputation: 916

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.

Upvotes: 6

MRB
MRB

Reputation: 3812

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();
});

Upvotes: 4

Adrian McCarthy
Adrian McCarthy

Reputation: 47954

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.

Upvotes: 4

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