How efficient is std::string compared to null-terminated strings?

Question

I've discovered that std::strings are very slow compared to old-fashioned null-terminated strings, so much slow that they significantly slow down my overall program by a factor of 2.

I expected STL to be slower, I didn't realise it was going to be this much slower.

I'm using Visual Studio 2008, release mode. It shows assignment of a string to be 100-1000 times slower than char* assignment (it's very difficult to test the run-time of a char* assignment). I know it's not a fair comparison, a pointer assignment versus string copy, but my program has lots of string assignments and I'm not sure I could use the "const reference" trick in all places. With a reference counting implementation my program would have been fine, but these implementations don't seem to exist anymore.

My real question is: why don't people use reference counting implementations anymore, and does this mean we all need to be much more careful about avoiding common performance pitfalls of std::string?

My full code is below.

#include <string>
#include <iostream>
#include <time.h>

using std::cout;

void stop()
{
}

int main(int argc, char* argv[])
{
    #define LIMIT 100000000
    clock_t start;
    std::string foo1 = "Hello there buddy";
    std::string foo2 = "Hello there buddy, yeah you too";
    std::string f;
    start = clock();
    for (int i=0; i < LIMIT; i++) {
        stop();
        f = foo1;
        foo1 = foo2;
        foo2 = f;
    }
    double stl = double(clock() - start) / CLOCKS\_PER\_SEC;

    start = clock();
    for (int i=0; i < LIMIT; i++) {
        stop();
    }
    double emptyLoop = double(clock() - start) / CLOCKS_PER_SEC;

    char* goo1 = "Hello there buddy";
    char* goo2 = "Hello there buddy, yeah you too";
    char *g;
    start = clock();
    for (int i=0; i < LIMIT; i++) {
        stop();
        g = goo1;
        goo1 = goo2;
        goo2 = g;
    }
    double charLoop = double(clock() - start) / CLOCKS_PER_SEC;
    cout << "Empty loop = " << emptyLoop << "\n";
    cout << "char* loop = " << charLoop << "\n";
    cout << "std::string = " << stl << "\n";
    cout << "slowdown = " << (stl - emptyLoop) / (charLoop - emptyLoop) << "\n";
    std::string wait;
    std::cin >> wait;
    return 0;
}

Answer 1

Well there are definitely known problems regarding the performance of strings and other containers. Most of them have to do with temporaries and unnecessary copies.

It's not too hard to use it right, but it's also quite easy to Do It Wrong. For example, if you see your code accepting strings by value where you don't need a modifiable parameter, you Do It Wrong:

// you do it wrong
void setMember(string a) {
    this->a = a; // better: swap(this->a, a);
}

You better had taken that by const reference or done a swap operation inside, instead of yet another copy. Performance penalty increases for a vector or list in that case. However, you are right definitely that there are known problems. For example in this:

// let's add a Foo into the vector
v.push_back(Foo(a, b));

We are creating one temporary Foo just to add a new Foo into our vector. In a manual solution, that might create the Foo directly into the vector. And if the vector reaches its capacity limit, it has to reallocate a larger memory buffer for its elements. What does it do? It copies each element separately to their new place using their copy constructor. A manual solution might behave more intelligent if it knows the type of the elements before-hand.

Another common problem is introduced temporaries. Have a look at this

string a = b + c + e;

There are loads of temporaries created, which you might avoid in a custom solution that you actually optimize onto performance. Back then, the interface of std::string was designed to be copy-on-write friendly. However, with threads becoming more popular, transparent copy on write strings have problems keeping their state consistent. Recent implementations tend to avoid copy on write strings and instead apply other tricks where appropriate.

Most of those problems are solved however for the next version of the Standard. For example instead of push_back, you can use emplace_back to directly create a Foo into your vector

v.emplace_back(a, b);

And instead of creating copies in a concatenation above, std::string will recognize when it concatenates temporaries and optimize for those cases. Reallocation will also avoid making copies, but will move elements where appropriate to their new places.

For an excellent read, consider Move Constructors by Andrei Alexandrescu.

Sometimes, however, comparisons also tend to be unfair. Standard containers have to support the features they have to support. For example if your container does not keep map element references valid while adding/removing elements from your map, then comparing your "faster" map to the standard map can become unfair, because the standard map has to ensure that elements keep being valid. That was just an example, of course, and there are many such cases that you have to keep in mind when stating "my container is faster than standard ones!!!".