Converting std::string from const_iterator to char*?

Question

I have an iterator of type:

std::map<int, std::string>::const_iterator

and as you can see the map is storing std::strings.

I am trying to extract a value using:

x.second.c_str()

and pass it to a function to call the regex.h function regexec(), which accepts a char*. What is the best way to do this? I am getting basic_string compilation errors from this approach.

Update:

std::map<int, std::string> map;
.
.
.
std::map<int, std::string>::const_iterator f = map.find("something");
if (f != map.end()) {
    my_func(f->second.c_str());   
.
.
.
.

void my_func(char* c){
    std::cout << c << std::endl;   //This causes a segmentation fault
}

What is x? If it is an iterator you should do x->second.c_str() — Alan, Jun 03 '14 at 14:35
Please provide an [SSCCE](http://sscce.org/), so that others can understand your exact problem. — Csq, Jun 03 '14 at 14:37
Duplicate of http://stackoverflow.com/questions/347949/convert-stdstring-to-const-char-or-char?rq=1 — Bruno Ferreira, Jun 03 '14 at 14:39
**1)** `regexec` seems to accept a `const char*`, which is a huge difference. **2)** If you go to the mechanic and say "something's wrong with my car" he wont be able to help. Same applies here: Show the ~~car~~ code and tell us exactly *what* is wrong with it, i.e. provide the error messages. — Arne Mertz, Jun 03 '14 at 14:41
That is **not** all the code, or you have a very strange compiler. — Arne Mertz, Jun 03 '14 at 14:43
First you have written about `regexec` and compilation errors related to `basic_string`, but now your code shows another function, does not mention `regexec` and has a comment about segfaults. What is your actual problem? — Arne Mertz, Jun 03 '14 at 14:46
Are you sure that map.find("something") is correct. The key in your map is an int. — Alan, Jun 03 '14 at 14:47

score 0 · Answer 1 · answered Jun 03 '14 at 14:43

You should do

int error=regexec(&regex,cppstring.c_str(),nmatch,pmatch,eflags);

The second argument of regexec() and the return value of std::string::c_str() are both (const char *), so that's probably not the cause of your issue.

You should post the compiler error message and the part of the code that you suspect to be causing the problem.

score 0 · Answer 2 · answered Jun 03 '14 at 15:19

Here's a very wild guess, though I could be mislead by the contradictory and incomplete information given:

I assume you have a function that takes a char* as parameter, e.g. like the my_func provided in the question.
I further assume you try to dereference a const_iterator of some kind, ultimately getting a reference to a const string, and call c_str() on it.
The next assumption is that you want to pass the result of that call to your function.

That would look somewhat like this:

void my_func(char*);

int main() {
  std::map<int, std::string> theMap;
  // ...
  std::map<int, std::string>::const_iterator cit = theMap.find(3);
  assert(cit != theMap.end());


  my_func(cit->second.c_str()); //problematic line
}

In this case, you should get a compilation error, because c_str() returns a const char*. while your function expects a char*, and the two are not compatible.

However, my assumptions have several flaws:

The error should be about the conversion from const char* to char* and should not have to do anything with basic_string
It has nothing to do with regexec (but neither has your example code)
It cannot lead to segfaults, since it does not compile (but that contradiction is already present in your question)

If you shed some more light on the question, I will be able to give more specific hints on what is going wrong.

Converting std::string from const_iterator to char*?

2 Answers2