How do I concatenate multiple C++ strings on one line?

Question

#include <sstream>
#include <string>

std::stringstream ss;
ss << "Hello, world, " << myInt << niceToSeeYouString;
std::string s = ss.str();

Take a look at this Guru Of The Week article from Herb Sutter: The String Formatters of Manor Farm

In 5 years nobody has mentioned .append?

#include <string>

std::string s;
s.append("Hello world, ");
s.append("nice to see you, ");
s.append("or not.");

s += "Hello world, " + "nice to see you, " + "or not.";

Those character array literals are not C++ std::strings - you need to convert them:

s += string("Hello world, ") + string("nice to see you, ") + string("or not.");

To convert ints (or any other streamable type) you can use a boost lexical_cast or provide your own function:

template <typename T>
string Str( const T & t ) {
   ostringstream os;
   os << t;
   return os.str();
}

You can now say things like:

string s = string("The meaning is ") + Str( 42 );

Your code can be written as¹,

s = "Hello world," "nice to see you," "or not."

...but I doubt that's what you're looking for. In your case, you are probably looking for streams:

std::stringstream ss;
ss << "Hello world, " << 42 << "nice to see you.";
std::string s = ss.str();

---

¹ "can be written as" : This only works for string literals. The concatenation is done by the compiler.

Using C++14 user defined literals and std::to_string the code becomes easier.

using namespace std::literals::string_literals;
std::string str;
str += "Hello World, "s + "nice to see you, "s + "or not"s;
str += "Hello World, "s + std::to_string(my_int) + other_string;

Note that concatenating string literals can be done at compile time. Just remove the +.

str += "Hello World, " "nice to see you, " "or not";

In C++20 you'll be able to do:

auto s = std::format("{}{}{}", "Hello world, ", myInt, niceToSeeYouString);

Until then you could do the same with the {fmt} library:

auto s = fmt::format("{}{}{}", "Hello world, ", myInt, niceToSeeYouString);

Disclaimer: I'm the author of {fmt}.

To offer a solution that is more one-line-ish: A function concat can be implemented to reduce the "classic" stringstream based solution to a single statement. It is based on variadic templates and perfect forwarding.

---

Usage:

std::string s = concat(someObject, " Hello, ", 42, " I concatenate", anyStreamableType);

---

Implementation:

void addToStream(std::ostringstream&)
{
}

template<typename T, typename... Args>
void addToStream(std::ostringstream& a_stream, T&& a_value, Args&&... a_args)
{
    a_stream << std::forward<T>(a_value);
    addToStream(a_stream, std::forward<Args>(a_args)...);
}

template<typename... Args>
std::string concat(Args&&... a_args)
{
    std::ostringstream s;
    addToStream(s, std::forward<Args>(a_args)...);
    return s.str();
}