I am converting a character to a string by concatenating it with an empty string (""). But it results in undefined behaviour or garbage characters in the resultant string. Why so?
char c = 'a';
string s = ""+c;
cout<<s<<" "<<s.size()<<"\n";
2 Answers
Let's look at your snippet, one statement or a line at a time.
char c = 'a';
This is valid, assigning a character literal to a variable of type char.
Note: since c is not changed after this statement, you may want to declare the definition as const.
string s = ""+c;
Let's refactor:
std::string s = ("" + c);
Let's add type casting to make the point more clear:
std::string s = ((const char *) "" + (char) c);
The order of operations is resolve all expressions on the right hand side (rhs) of the assignment operator before assigning to the string variable.
There is no operator+() in C++ that takes a const char * and a single character.
The compiler is looking for this function: operator+(const char *, char).
This is the primary reason for the compilation errors.
cout<<s<<" "<<s.size()<<"\n";
The string assignment and creation failed, thus s is empty and s.size() is zero.
3 Comments
Yksisarvinen
There is no operator+() in C++ that takes a const char * and a single character. but there is one that takes a pointer and integer. There are no compiler errors here, just UB.
Thomas Matthews
So, the compiler tries to find the "best match" for the operator, but realizes that
(char) c can be converted to an int and applies the operator+(char *, int).
Pete Becker
@ThomasMatthews -- there's no "best match" involved; "best match" is the algorithm for choosing one of several viable overloads of a function. That's not what's going on here; it's straight C, with a pointer and an integer value.
An interesting issue, which looks like an issue which the compiler does not detect at compilation.
I have tried this with GNU C++ 10.2 on Cygwin and your code snippet generates this.
0200312 (Fedora Cygwin 9.3.0-1) 31
if you replace char c = 'c'; with char c = '\0'; the output is
0
So looks like some kind of memory violation issue.
As you probably know, the standard method of appending a char is to use push_back
string s = "";
s.push_back(c);
cout << s << " " << s.size() << "\n";
Alternatively you can use the operator +=
string s = "";
s += c;
cout << s << " " << s.size() << "\n";
""is not astringstring s = ""s + c; (unless you're truly limited to C++11). Then read this.""isconst char[1], notstd::string. The naming is a bit confusing, I admit.char.