how to convert vector<string> to const char**

.gitignore

@@ -22,3 +22,4 @@ future
 return-type-deduction
 division
 poly
+ohai-const-strings

CMakeLists.txt

@@ -20,6 +20,7 @@ add_executable(bad_alloc bad_alloc.cpp)
 add_executable(rtti rtti.cpp)
 add_executable(poly poly.cpp)
 add_executable(division division.cpp)
+add_executable(ohai-const-strings ohai-const-strings.cpp)
 
 add_executable(future future.cpp)
 set_target_properties(future PROPERTIES LINK_FLAGS "-pthread")

ohai-const-strings.cpp

@@ -0,0 +1,68 @@
+// how to convert std::vector<std::string> to const char**?
+
+#include <string.h>
+
+#include <iostream>
+#include <string>
+#include <vector>
+
+void print_strings(const std::vector<std::string> strings) {
+  using std::cout;
+  using std::endl;
+
+  cout << "== Printing some std::vector<std::string> strings:" << endl;
+  for (const std::string &s : strings) {
+    cout << s << endl;
+  }
+}
+
+const char** convert_to_c(const std::vector<std::string> strings) {
+  // Note: perfectly fine to "new" const char*.
+  const char** strings_c = new const char*[strings.size()+1];
+  for (size_t i = 0; i < strings.size(); ++i) {
+    // XXX No need to copy the string here.
+    char* string_c = new char[strings.at(i).size()+1];
+    strncpy(string_c, strings.at(i).c_str(), strings.at(i).size()+1);
+    strings_c[i] = string_c;
+  }
+  strings_c[strings.size()] = NULL;
+
+  return strings_c;
+}
+
+void free_c(const char** strings_c) {
+  for (size_t i = 0; strings_c[i] != NULL; ++i) {
+    delete[] strings_c[i];
+  }
+  delete[] strings_c;
+}
+
+void print_strings_c(const char** strings_c) {
+  using std::cout;
+  using std::endl;
+
+  cout << "== Printing some const char** strings:" << endl;
+  for (size_t i = 0; strings_c[i] != NULL; ++i) {
+    cout << strings_c[i] << endl;
+  }
+}
+
+void foo(const char* string) {
+  printf("foo: %s\n", string);
+}
+
+int main() {
+  foo("foo");
+  char* bar = new char[4];
+  strncpy(bar, "bar", 4);
+  foo(bar);
+  delete[] bar;
+
+  std::vector<std::string> some_strings = { "just", "some", "strings", "here" };
+  print_strings(some_strings);
+
+  const char **some_strings_c = convert_to_c(some_strings);
+
+  print_strings_c(some_strings_c);
+  free_c(some_strings_c);
+}