Most optimized way of concatenation in strings
In addition to other answers...
I made extensive benchmarks about this problem some time ago, and came to the conclusion that the most efficient solution (GCC 4.7 & 4.8 on Linux x86 / x64 / ARM) in all use cases is first to reserve() the result string with enough space to hold all the concatenated strings, and then only append() them (or use operator +=(), that makes no difference).
Unfortunately it seems I deleted that benchmark so you only have my word (but you can easily adapt Mats Petersson's benchmark to verify this by yourself, if my word isn't enough).
In a nutshell:
const string space = " ";
string result;
result.reserve(5 + space.size() + 5);
result += "hello";
result += space;
result += "world";
Depending on the exact use case (number, types and sizes of the concatenated strings), sometimes this method is by far the most efficient, and other times it is on par with other methods, but it is never worse.
Problem is, this is really painful to compute the total required size in advance, especially when mixing string literals and std::string (the example above is clear enough on that matter, I believe). The maintainability of such code is absolutely horrible as soon as you modify one of the literals or add another string to be concatenated.
One approach would be to use sizeof to compute the size of the literals, but IMHO it creates as much mess than it solves, the maintainability is still terrible:
#define STR_HELLO "hello"
#define STR_WORLD "world"
const string space = " ";
string result;
result.reserve(sizeof(STR_HELLO)-1 + space.size() + sizeof(STR_WORLD)-1);
result += STR_HELLO;
result += space;
result += STR_WORLD;
A usable solution (C++11, variadic templates)
I finally settled for a set of variadic templates that efficiently take care of calculating the string sizes (eg. the size of string literals is determined at compile time), reserve() as needed, and then concatenate everything.
Here it is, hope this is useful:
namespace detail {
template<typename>
struct string_size_impl;
template<size_t N>
struct string_size_impl<const char[N]> {
static constexpr size_t size(const char (&) [N]) { return N - 1; }
};
template<size_t N>
struct string_size_impl<char[N]> {
static size_t size(char (&s) [N]) { return N ? strlen(s) : 0; }
};
template<>
struct string_size_impl<const char*> {
static size_t size(const char* s) { return s ? strlen(s) : 0; }
};
template<>
struct string_size_impl<char*> {
static size_t size(char* s) { return s ? strlen(s) : 0; }
};
template<>
struct string_size_impl<std::string> {
static size_t size(const std::string& s) { return s.size(); }
};
template<typename String> size_t string_size(String&& s) {
using noref_t = typename std::remove_reference<String>::type;
using string_t = typename std::conditional<std::is_array<noref_t>::value,
noref_t,
typename std::remove_cv<noref_t>::type
>::type;
return string_size_impl<string_t>::size(s);
}
template<typename...>
struct concatenate_impl;
template<typename String>
struct concatenate_impl<String> {
static size_t size(String&& s) { return string_size(s); }
static void concatenate(std::string& result, String&& s) { result += s; }
};
template<typename String, typename... Rest>
struct concatenate_impl<String, Rest...> {
static size_t size(String&& s, Rest&&... rest) {
return string_size(s)
+ concatenate_impl<Rest...>::size(std::forward<Rest>(rest)...);
}
static void concatenate(std::string& result, String&& s, Rest&&... rest) {
result += s;
concatenate_impl<Rest...>::concatenate(result, std::forward<Rest>(rest)...);
}
};
} // namespace detail
template<typename... Strings>
std::string concatenate(Strings&&... strings) {
std::string result;
result.reserve(detail::concatenate_impl<Strings...>::size(std::forward<Strings>(strings)...));
detail::concatenate_impl<Strings...>::concatenate(result, std::forward<Strings>(strings)...);
return result;
}
The only interesting part, as far as the public interface is concerned, is the very last template<typename... Strings> std::string concatenate(Strings&&... strings) template. Usage is straightforward:
int main() {
const string space = " ";
std::string result = concatenate("hello", space, "world");
std::cout << result << std::endl;
}
With optimizations turned on, any decent compiler should be able to expand the concatenate call to the same code as my first example where I manually wrote everything. As far as GCC 4.7 & 4.8 are concerned, the generated code is pretty much identical as well as the performance.
Here is a small test suite:
#include <iostream>
#include <string>
#include <chrono>
#include <sstream>
int main ()
{
typedef std::chrono::high_resolution_clock clock;
typedef std::chrono::duration<float, std::milli> mil;
std::string l_czTempStr;
std::string s1="Test data1";
auto t0 = clock::now();
#if VER==1
for (int i = 0; i < 100000; ++i)
{
l_czTempStr = s1 + "Test data2" + "Test data3";
}
#elif VER==2
for (int i = 0; i < 100000; ++i)
{
l_czTempStr = "Test data1";
l_czTempStr += "Test data2";
l_czTempStr += "Test data3";
}
#elif VER==3
for (int i = 0; i < 100000; ++i)
{
l_czTempStr = "Test data1";
l_czTempStr.append("Test data2");
l_czTempStr.append("Test data3");
}
#elif VER==4
for (int i = 0; i < 100000; ++i)
{
std::ostringstream oss;
oss << "Test data1";
oss << "Test data2";
oss << "Test data3";
l_czTempStr = oss.str();
}
#endif
auto t1 = clock::now();
std::cout << l_czTempStr << '\n';
std::cout << mil(t1-t0).count() << "ms\n";
}
On coliru:
Compile with the following:
clang++ -std=c++11 -O3 -DVER=1 -Wall -pedantic -pthread main.cpp
21.6463ms
-DVER=2
6.61773ms
-DVER=3
6.7855ms
-DVER=4
102.015ms
It looks like 2), += is the winner.
(Also compiling with and without -pthread seems to affect the timings)