I think it's insane that this question is over a year old and the only answer implies pumping several megabytes into a program that otherwise wouldn't need it.
You can use platform-specific preprocessor directives to achieve what you want.

#include <string>

#ifdef _WIN32
static int platform = 1;
#elif _WIN64
static int platform= 1;
#elif __linux__
static int platform = 2;
#elif __APPLE__
static int platform = 3;
#else
static int platform = 0;
#endif

int main(int argc, char *argv[])
{
  std::string str;
  std::string p = "https://crouton.net";
  if(platform) switch(platform)
    {
    case 1:
      str = "explorer";
      break;
    case 2:
      str = "xdg-open";
      break;
    case 3:
      str = "open";
      break;
    default:
      return 1; // Should never happen on the 3 defined platforms
    }
  str.append(" " + p);
  std::system(str.data());
  return 0;
}

It is important to keep in mind that this code can be run on more than just Windows, Mac and Linux. The usage of __APPLE__ is based on the assumption that what you're developing is a desktop program -- it likely wouldn't work on iOS, where the definition is still valid.
This example will only work for URLs (e.g. you can't replace the URL with the name of an application). If you don't know the path of what you're trying to open, you can attempt to get it using other platform-specific commands, but it's never guaranteed that you will find it even if the user has it installed. It is a very platform-specific thing because Windows applications often don't utilize the PATH, as they're expected far less to be invoked on the command line.

可以使用Qt cross-platform C++ framework，具体启动进程可以使用QProcess启动程序，也可以使用QDesktopServices：

#include <QtWidgets/QApplication>

#include <QProcess>
#include <QDesktopServices>
#include <QUrl>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    // You can specify the path in some config file
    const QString pathToApp = "/usr/bin/google-chrome-stable";

    // Open an application by given path
    // No need to add "open" or some other OS-specific prefix
    QProcess::startDetached(pathToApp);

    // OR
    // Open target web address in a default system browser in any OS
    QDesktopServices::openUrl(QUrl("www.google.com"));

    return a.exec();
}