如何在C ++中使用转换说明符？

Question

In Python I can do this:

print("Hello, %s!" % username)

How do I in C++ do the similar thing? I can do this, but it's probably not a good practice:

void welcome_message(std::string username) {
    std::cout << "Hello, " << username << "!" <<std::endl;
}

Answer 1

For simple code, you may use std::cout . But this is not very good for localisation and looks kind of ugly.

Boost.Format fixes this by providing functionality very similar to that old Python 2 string formatting feature you demonstrate in your question (which was superseded by str.format() ). It's a little like C's printf , except safe.

Here's an example:

#include <string>
#include <iostream>
#include <boost/format.hpp>

void welcome_message(const std::string& username)
{
    std::cout << boost::format("Hello, %s!\n") % username;
}

int main()
{
    welcome_message("Jim");
}

( live demo )

Answer 2

void welcome_message(const std::string& username) {
    std::cout << "Hello, " << username << "!" <<std::endl;
}

is an extremely good thing to do in C++ unless you're doing something particularly performance-critical. Relying on the heavily overloaded << for std::ostream is good practice. It's what it's there for.

Note I've changed the function prototype to avoid an unnecessary string copy. The printf way is littered with dangerous corner-cases: you need to get the formatters exactly correct else you risk the behaviour of your program being undefined.

The only criticism I can levy on this is your use of std::endl . Use "!\\n" instead.

Answer 3

You can always C's printf:

#include <cstdio>

void welcome_message(const std::string &username) {
    printf("Hello, %s\n", username.c_str());
}

Notice, though, that you have to convert username to a C string using c_str() .

As mentioned, using cout and << is pretty standard, but many people argue against it for many reasons (eg: iternationalization, readability). So, if you prefer string formatting, there's your option.