Converting a vector<char *> to char ** array? [duplicate] - c++

Question

This question already has answers here:
convert vector<string> into char** C++
(4 answers)
Closed 2 years ago.
I'm trying to convert a vector<char * > into a char** array. When I print parsed[0], I get garbage. How would I fix this?
char **parse(vector<string> arr){
vector<char*> res;
for(int i = 1; i < arr.size(); i++){
string s = arr.at(i);
char temp[s.length() + 1];
s.copy(temp, s.length()+1);
temp[s.length()] = '\0';
res.push_back(temp);
}
res.push_back(NULL);
char **parsed = res.data();
cout << parsed[0] << endl;
return parsed;
}

Answer 1

Just going by the title calling vector<char*>.data() seems to be the most straightforward way:
#include <type_traits>
#include <vector>
int main()
{
std::vector<char*> chars{};
static_assert(std::is_same_v<char**, decltype(chars.data())>);
}

Answer 2

This seems like a really convoluted method of just doing:
#include <algorithm>
#include <vector>
#include <iostream>
#include <string>
std::vector<const char*> to_ptrs(const std::vector<std::string>& arr) {
std::vector<const char*> res;
for (auto& s : arr) {
res.emplace_back(s.c_str());
}
return res;
}
int main() {
std::vector<std::string> input = {
"test",
"example",
"of",
"strings"
};
auto ptr_vec = to_ptrs(input); // To avoid dangling temporary
const char** result = ptr_vec.data();
for (size_t i = 0; i < input.size(); ++i) {
std::cout << result[i] << std::endl;
}
return 0;
}
Or if you prefer a C++20 solution with std::transform:
std::vector<const char*> to_ptrs(const std::vector<std::string>& arr) {
std::vector<const char*> res;
std::transform(arr.begin(), arr.end(), std::back_inserter(res), [](const std::string& s) -> const char* { return s.c_str(); });
return res;
}
Note conversion to char** is not possible because the result of c_str() is const char*.
Note: Any alterations to the original strings or string data invalidates these pointers.