Splitting C++ Strings Onto Multiple Lines (Code Syntax, Not Parsing)

问题

Not to be confused with how to split a string parsing wise, e.g.:
Split a string in C++?

I am a bit confused as to how to split a string onto multiple lines in c++.

This sounds like a simple question, but take the following example:

#include <iostream>
#include <string>
main() {
  //Gives error
  std::string my_val ="Hello world, this is an overly long string to have" +
    " on just one line";
  std::cout << "My Val is : " << my_val << std::endl;

  //Gives error
  std::string my_val ="Hello world, this is an overly long string to have" &
    " on just one line";  
  std::cout << "My Val is : " << my_val << std::endl;
}

I realize that I could use the std::string append() method, but I was wondering if there was any shorter/more elegant (e.g. more pythonlike, though obviously triple quotes etc. aren't supported in c++) way to break strings in c++ onto multiple lines for sake of readability.

One place where this would be particularly desirable is when you're passing long string literals to a function (for example a sentence).

回答1:

Don't put anything between the strings. Part of the C++ lexing stage is to combine adjacent string literals (even over newlines and comments) into a single literal.

#include <iostream>
#include <string>
main() {
  std::string my_val ="Hello world, this is an overly long string to have" 
    " on just one line";
  std::cout << "My Val is : " << my_val << std::endl;
}

Note that if you want a newline in the literal, you will have to add that yourself:

#include <iostream>
#include <string>
main() {
  std::string my_val ="This string gets displayed over\n" 
    "two lines when sent to cout.";
  std::cout << "My Val is : " << my_val << std::endl;
}

If you are wanting to mix a #defined integer constant into the literal, you'll have to use some macros:

#include <iostream>
using namespace std;

#define TWO 2
#define XSTRINGIFY(s) #s
#define STRINGIFY(s) XSTRINGIFY(s)

int main(int argc, char* argv[])
{
    std::cout << "abc"   // Outputs "abc2DEF"
        STRINGIFY(TWO)
        "DEF" << endl;
    std::cout << "abc"   // Outputs "abcTWODEF"
        XSTRINGIFY(TWO) 
        "DEF" << endl;
}

There's some weirdness in there due to the way the stringify processor operator works, so you need two levels of macro to get the actual value of TWO to be made into a string literal.

回答2:

Are they both literals? Separating two string literals with whitespace is the same as concatenation: "abc" "123" is the same as "abc123". This applies to straight C as well as C++.

回答3:

I don't know if it is an extension in GCC or if it is standard, but it appears you can continue a string literal by ending the line with a backslash (just as most types of lines can be extended in this manor in C++, e.g. a macro spanning multiple lines).

#include <iostream>
#include <string>

int main ()
{
    std::string str = "hello world\
    this seems to work";

    std::cout << str;
    return 0;
}

来源：https://stackoverflow.com/questions/3859157/splitting-c-strings-onto-multiple-lines-code-syntax-not-parsing

标签

c++

string

syntax

coding-style

readability