Compiler error message "error: could not convert from 'std::string* {aka std::basic_string<char>*}' to 'std::string {aka std::basic_string<char>}'|"

I'm trying to make a function that writes a file, but I'm having issues passing a string as a parameter.

void writeFile(string filename, string letters, int size)
{
     ofstream outputfile("output.txt");
     outputfile << letters;
     outputfile.close();
 }

int main()
{
    string letters[] = {"u", "l", "s", "n", "m", "z", "a", "p", "b"};

    int size = 9;

    string filename = "Inputfile.txt";

    writeFile(inputfilename.c_str(), letters, size);

}

And I get this error:

error: could not convert from 'std::string* {aka std::basic_string*}' to 'std::string {aka std::basic_string}'|

An array of string items is passed as actual argument, where the formal argument is a single string.

You could replace the array with a single string.

Or with a set, or whatever suits the purpose, but then you'll have to change the called function accordingly.

The error mentions std::string* instead of an array, like std::string[9], because the array expression decays to an expression denoting a pointer to the first item, which it does because the array is not being bound to a reference or passed to sizeof or anything where the expression's array nature would have to be preserved.