Why allow concatenation of string literals?

Question

I recently got bit by a subtle bug.

char ** int2str = {
   "zero", // 0
   "one",  // 1
   "two"   // 2
   "three",// 3
   nullptr };

assert( values[1] == "one"_s ); // passes
assert( values[2] == "two"_s ); // fails

If you have godlike code review powers you'll notice I forgot the , after "two".

After the considerable effort to find that bug I've got to ask why would anyone ever want this behavior?

I can see how this might be useful for macro magic, but then why is this a "feature" in a modern language like python?

Have you ever used string literal concatenation in production code?

Answer 1

I certainly have in both C and C++. Offhand, I don't see much relationship between its utility and how "modern" the language is.

Answer 2

So that you can split long string literals across lines.

And yes, I've seen it in production code.

Answer 3

Cases where this can be useful:

• Generating strings including components defined by the preprocessor (this is perhaps the largest use case in C, and it's one I see very, very frequently).
• Splitting string constants over multiple lines

To provide a more concrete example for the former:

// in version.h
#define MYPROG_NAME "FOO"
#define MYPROG_VERSION "0.1.2"

// in main.c
puts("Welcome to " MYPROG_NAME " version " MYPROG_VERSION ".");

Answer 4

It's a great feature that allows you to combine preprocessor strings with your strings.

// Here we define the correct printf modifier for time_t
#ifdef TIME_T_LONG
    #define TIME_T_MOD "l"
#elif defined(TIME_T_LONG_LONG)
    #define TIME_T_MOD "ll"
#else
    #define TIME_T_MOD ""
#endif

// And he we merge the modifier into the rest of our format string
printf("time is %" TIME_T_MOD "u\n", time(0));

Answer 5

Sure, it's the easy way to make your code look good:

char *someGlobalString = "very long "
                         "so broken "
                         "onto multiple "
                         "lines";

The best reason, though, is for weird printf formats, like type forcing:

uint64_t num = 5;
printf("Here is a number:  %"PRIX64", what do you think of that?", num);

There are a bunch of those defined, and they can come in handy if you have type size requirements. Check them all out at this link. A few examples:

PRIo8 PRIoLEAST16 PRIoFAST32 PRIoMAX PRIoPTR

Answer 6

I'm not sure about other programming languages, but for example C# doesn't allow you to do this (and I think this is a good thing). As far as I can tell, most of the examples that show why this is useful in C++ would still work if you could use some special operator for string concatenation:

string someGlobalString = "very long " +
                          "so broken " +
                          "onto multiple " +
                          "lines"; 

This may not be as comfortable, but it is certainly safer. In your motivating example, the code would be invalid unless you added either , to separate elements or + to concatenate strings...

Answer 7

From the python lexical analysis reference, section 2.4.2:

This feature can be used to reduce the number of backslashes needed, to split long strings conveniently across long lines, or even to add comments to parts of strings

http://docs.python.org/reference/lexical_analysis.html