compile time string concatenation using constexpr

Question

#include <iostream> #include <string> constexpr std::string appendStringC(std::string s) { return s + " Constexpr func"; } std::string appendString(std::string s) { return s + " Regular func"; } int main() { std::string s1 = "String 1"; std::string s2 = "String 2"; std::cout << std::endl << appendStringC(s1) << std::endl << appendString(s2) << std::endl; return 0; }

I'm trying to perform compile time string prefix/postfix concatenation operation using constexpr. However this example is producing following errors:

const_string_generation.cpp: In function ‘constexpr std::__cxx11::string appendStringC(std::__cxx11::string)’: const_string_generation.cpp:4:23: error: invalid type for parameter 1 of constexpr function ‘constexpr std::__cxx11::string appendStringC(std::__cxx11::string)’ constexpr std::string appendStringC(std::string s) ^ In file included from /usr/include/c++/5.3.0/string:52:0, from /usr/include/c++/5.3.0/bits/locale_classes.h:40, from /usr/include/c++/5.3.0/bits/ios_base.h:41, from /usr/include/c++/5.3.0/ios:42, from /usr/include/c++/5.3.0/ostream:38, from /usr/include/c++/5.3.0/iostream:39, from const_string_generation.cpp:1: /usr/include/c++/5.3.0/bits/basic_string.h:71:11: note: ‘std::__cxx11::basic_string<char>’ is not literal because: class basic_string ^ /usr/include/c++/5.3.0/bits/basic_string.h:71:11: note: ‘std::__cxx11::basic_string<char>’ has a non-trivial destructor const_string_generation.cpp:4:23: error: invalid return type ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ of constexpr function ‘constexpr std::__cxx11::string appendStringC(std::__cxx11::string)’ constexpr std::string appendStringC(std::string s)

since std::string isn't a literal. I'm looking for an easy way to do this and I don't care about backward compatiblity for this very example. Ideone : Link

You can't, since std::string generally uses dynamic allocation internally (not very constexpr-friendly). However, you can concatenate two or more string literals by just placing them in sequence. — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Jan 4, 2016 at 11:37
@Cheersandhth.-Alf yes but I've lots of long strings that I need to concatenate in front or sometimes back of strings. Kind of like escape codes, so manually placing and editing each one of them might get clumsy soon. — Abhinav Gauniyal
– Abhinav Gauniyal, Commented Jan 4, 2016 at 11:53
You could use char sequence, something like stackoverflow.com/a/22067775/2684539 — Jarod42
– Jarod42, Commented Jan 4, 2016 at 12:08
@Jarod42 that's too much trouble, I would just throw a hacky macro approach if nothing works. — Abhinav Gauniyal
– Abhinav Gauniyal, Commented Jan 4, 2016 at 12:31

Paul Evans · Accepted Answer · 2016-01-04 11:47:10Z

std:string::operator+() isn't a constexpr, in fact it's usually implemented in a very dynamic manner that relies on heap memory. You can append static string constants like this:

"append this string " " to this string"

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