Is it possible to convert std::vector<std::string> to char* [] in one line?
Assume that I have this:
std::vector<std::string> my_vector;
And I add lots of text to it:
my_vector.push_back("A");
my_vector.push_back("AB");
my_vector.push_back("ABC");
Then I want to create a char * [] array from my_vector. How can that be done? Is there any smart functionality in C++ that can do this?
The goal is to have:
const char * my_array_cstr[] = {"A", "AB", "ABC"};
Sort of.
You can create something that (at least inside of the C++ domain) looks and acts like an array of pointers in one line. Since you haven't told us what you're hoping to accomplish by this, it's an open question whether it suits your purposes or not.
#include <vector>
#include <string>
#include <ranges>
int main() {
// some input to process
std::vector<std::string> input{ "A", "AB", "ABC"};
// do the transform in one line
auto ptrs = input | std::views::transform([](auto &s) { return s.c_str(); });
// use our "array of pointers":
for (auto i : ptrs)
printf("%s\n", i);
}
In theory, if you need something that's more like a real array of pointers (e.g., you can pass its address to a C function that expects the address of the beginning of an array) you can use a C++23 feature to do that:
auto ptrs = input | std::views::transform([](auto &s) { return s.c_str(); }) | to<std::vector>();
In this case, ptrs will be an actual vector of pointers, so its .data() will give you an actual pointer to a contiguous, dynamically allocated block of memory filled with pointers.
As I write this (6 June, 2023) there's only one compiler that actually implements that though: Microsoft C/C++ 19.34 (or, presumably newer). But g++ and Clang are still both missing this, so depending on your target, it may be no problem at all, or it may be completely unacceptable.
If you need to go the next step after that, and create a real, actual array...then you're largely out of luck. Although some compilers don't enforce the requirement, C++ requires that the size of an array be specified as a compile-time constant, so there's no way to create one with the same number of elements as an arbitrary vector (though if you know a maximum size at compile time, you can create an array of that maximum size and just use it).
It's not possible to do that in one line, or rather in one expression. In general there shouldn't be a reason to convert a vector to an array, unless you want compatibility with some c library. But, considering that a vector is a contiguous storage type you can do this:
/* My vector */
std::vector<std::string> my_vector { "A", "AB", "ABC" };
/* Fill a temporary vector with `const char*`s */
std::vector<const char*> temp;
temp.reserve(my_vector.size());
for (auto& s : my_vector)
temp.push_back(s.c_str());
/* Get `temp`'s array */
const char** my_array = temp.data();
If the contents of my_vector are all string literals, you can use std::vector<const char*> from the start and call data() to get the array pointer if necessary.
Is it possible to convert std::string to char* [] in one line inside C++?
None that I know of.
You can allocate a block of memory according to the size of the array. Then initialize it to the desired values via a for loop in a separate statement.
One (not recommended) option to allocate the memory is new/delete[]:
const auto my_size = my_vector.size();
const char **my_array_cstr = new const char *[my_size];
for (int i = 0; i < my_size; ++i) my_array_cstr[i] = my_vector[i].c_str();
// Use my_array_cstr
delete[] my_array_cstr;
Another is a variable length array, but this is not standard and can only be used if your compiler supports it:
const auto my_size = my_vector.size();
const char *my_array_cstr[my_size];
for (int i = 0; i < my_size; ++i) my_array_cstr[i] = my_vector[i].c_str();
Depending on your use case, if the size of the vector will be known at compile time, you can use a fixed size stack array:
const constexpr auto my_size = 3;
const char *my_array_cstr[my_size];
for (int i = 0; i < my_size; ++i) my_array_cstr[i] = my_vector[i].c_str();
