Can I invoke the stream operator overload without a stream object?

Question

Suppose I want to throw with a string containing information about some object, but the object implementation only has an overload for the stream operator (<<) rather than a cast to string. I want to do something like this:

throw std::runtime_error("Error, encountered invalid value " + x);

where x is an instance of a type which has (<<) overloaded. The above does not work, however, because the + is not overloaded to a type compatible with const char*. If x were a string (or castable to a string) it would work, but instead I have to do this:

std::stringstream s;
s << "Error, encountered invalid value " << x;
throw std::runtime_error(s.str());

How can I get something as concise as the first example without adding any overloads or custom functions. Does the Standard Library provide some features that will help here?

Answer 1

You can delegate to a function:

template <typename T>
std::string stream(const T& x) {
    std::ostringstream ss;
    ss << x;
    return ss.str();
}

throw std::runtime_error("Error..." + stream(x));

Which is also what boost::lexical_cast does:

throw std::runtime_error("Error..." + boost::lexical_cast<std::string>(x));

Or you can use a temporary stream, which involves having to do a cast since operator<< conventionally returns just a basic_ostream<char>&:

throw std::runtime_error(
    static_cast<std::ostringstream&&>(std::ostringstream{} << "Error..." << x)
    .str() );

Or you can wrap that logic into a separate type which, when streamed, converts the result to a string, for amusement's sake:

struct ToStrT {
    friend std::string operator<<(std::ostream& os, ToStrT ) {
        return static_cast<std::ostringstream&&>(os).str();
    }
};

constexpr ToStrT ToStr{};

throw std::runtime_error(std::ostringstream{} << "Error..." << x << ToStr);