How do I concatenate "constant strings" and chars? - c++

Question

What is the "proper way" to do the following? (Note, I don't want to output the message to the screen (yet), the data needs to be stored in a variable.)
std::cout << "Enter a letter: ";
char input;
std::cin >> input;
std::string message = "Today's program was brought to you by the letter '" + input + "'.";
The code gives me the error message invalid operands of types const char* and const char [3] to binary operator+.
I understand why this message is occurring. When Googling for a solution, the results that come up recommend casting each item into a string in turn. However, this becomes impractical if you have to concatenate a dozen items:
std::string("x:") + x + std::string(", y:") + y + std::string(", width:") + width + std::string(", height:") + height + ...;
What is the "proper way" in c++ to concatenate strings, chars, char arrays, and any other data that is convertible, into a single string? Is there anything like Python's beautiful string formatting features in c++?

Answer 1

What you are trying to do won't work because C++ views your concatenation as an attempt to add several char pointers. If you explicitly cast the first element in the series to an std::string it should work.
Change your original code
string message = "Today's program was brought to you by the letter '" + input + "'.";
to this:
string message = std::string("Today's program was brought to you by the letter '")
+ input + "'.";
q.v. this SO post which discusses this problem in greater detail (though I don't know why it got closed as not being a real question).

Answer 2

There's several ways to do this, but one way that's a good balance between simplicity of implementation and convenience is to use a "formatter" class which wraps std::stringstream like so:
string message = formatter() << "Today's program was brought to you by the letter '" << input << "'.";
Where formatter can be defined very simply in a header file as follows:
#include <sstream>
class formatter {
public:
template <typename T>
formatter & operator<<(const T & o) {
stream_ << o;
return *this;
}
const std::string str() const { return stream_.str(); }
operator std::string() {
return stream_.str();
}
private:
std::ostringstream stream_;
};
What's going on there: If you try to use a temporary std::stringstream() instead of formatter() above, it doesn't work because
std::stringstream is not implicitly convertible to std::string
You can't even do it like this
std::string message = (std::stringstream() << "foo" << input << "bar").str(); because, std::stringstream returns std::ostream & from its stream operations (rather than std::stringstream &), and you cannot convert an ostream to a string in general.
The formatter class just lets you construct and use a stringstream all in one line with a minimum of boiler plate.