The problem is the two return 0; statements in your function. The function returns a std::string, which has no constructors that accept an int as input. But, it does have a constructor that accepts a const char * pointer, which 0 is implicitly convertible to. However, constructing a std::string with a null char * pointer is undefined behavior, and your implementation has chosen to throw a std::logic_error exception that you are not catching in your code.
In this case, I would simply return a blank string instead:
std::string myfunc(const std::string &input){
if (input.empty()) return “”;
for (int i = 0; i < input.size(); ++i){
char ch = input[i];
if ( !((ch >= ‘a’ && ch <= 'z') || (ch >= ‘0’ && ch <= '9')) ) return "";
}
return input;
}
The caller can then check if the return value is empty, if it wants to:
if (myfunc(input).empty())
// error, do something
else
// OK, do something else
Which would be better served with a function that returns a bool instead of a std::string:
bool isvalid(const std::string &input){
if (input.empty()) return false;
for (int i = 0; i < input.size(); ++i){
char ch = input[i];
if ( !((ch >= ‘a’ && ch <= 'z') || (ch >= ‘0’ && ch <= '9')) ) return false;
}
return true;
}
// if you still needed this function for something...
std::string myfunc(const std::string &input){
if (!isvalid(input)) return "";
return input;
}
if (!isvalid(input))
// error, do something
else
// OK, do something else
If you want to return false (or true) then you should change the return type of your function to bool
bool myfunc(std::string input) {
^^^^
Secondly if you mean to return false then that's what you should return
if (!input.size()) return false;
^^^^^
Returning 0 from a boolean function is not an error because 0 will automatically be converted to false, but clearly it's stylistically better to say what you mean.