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!!!".

Answer 2

It looks like you're misusing char* in the code you pasted. If you have

std::string a = "this is a";
std::string b = "this is b"
a = b;

you're performing a string copy operation. If you do the same with char*, you're performing a pointer copy operation.

The std::string assignment operation allocates enough memory to hold the contents of b in a, then copies each character one by one. In the case of char*, it does not do any memory allocation or copy the individual characters one by one, it just says "a now points to the same memory that b is pointing to."

My guess is that this is why std::string is slower, because it's actually copying the string, which appears to be what you want. To do a copy operation on a char* you'd need to use the strcpy() function to copy into a buffer that's already appropriately sized. Then you'll have an accurate comparison. But for the purposes of your program you should almost definitely use std::string instead.

Answer 3

When writing C++ code using any utility class (whether STL or your own) instead of eg. good old C null terminated strings, you need to rememeber a few things.

• If you benchmark without compiler optimisations on (esp. function inlining), classes will lose. They are not built-ins, even stl. They are implemented in terms of method calls.

• Do not create unnesessary objects.

• Do not copy objects if possible.

• Pass objects as references, not copies, if possible,

• Use more specialised method and functions and higher level algorithms. Eg.:

  std::string a = "String a"
  std::string b = "String b"
  
  // Use
  a.swap(b);
  
  // Instead of
  std::string tmp = a;
  a = b;
  b = tmp;
  

And a final note. When your C-like C++ code starts to get more complex, you need to implement more advanced data structures like automatically expanding arrays, dictionaries, efficient priority queues. And suddenly you realise that its a lot of work and your classes are not really faster then stl ones. Just more buggy.

Answer 4

You are most certainly doing something wrong, or at least not comparing "fairly" between STL and your own code. Of course, it's hard to be more specific without code to look at.

It could be that you're structuring your code using STL in a way that causes more constructors to run, or not re-using allocated objects in a way that matches what you do when you implement the operations yourself, and so on.

Answer 5

This test is testing two fundamentally different things: a shallow copy vs. a deep copy. It's essential to understand the difference and how to avoid deep copies in C++ since a C++ object, by default, provides value semantics for its instances (as with the case with plain old data types) which means that assigning one to the other is generally going to copy.

I "corrected" your test and got this:

char* loop = 19.921
string = 0.375
slowdown = 0.0188244

Apparently we should cease using C-style strings since they are soooo much slower! In actuality, I deliberately made my test as flawed as yours by testing shallow copying on the string side vs. strcpy on the :

#include <string>
#include <iostream>
#include <ctime>

using namespace std;

#define LIMIT 100000000

char* make_string(const char* src)
{
    return strcpy((char*)malloc(strlen(src)+1), src);
}

int main(int argc, char* argv[])
{
    clock_t start;
    string foo1 = "Hello there buddy";
    string foo2 = "Hello there buddy, yeah you too";
    start = clock();
    for (int i=0; i < LIMIT; i++)
        foo1.swap(foo2);
    double stl = double(clock() - start) / CLOCKS_PER_SEC;

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

The main point is, and this actually gets to the heart of your ultimate question, you have to know what you are doing with the code. If you use a C++ object, you have to know that assigning one to the other is going to make a copy of that object (unless assignment is disabled, in which case you'll get an error). You also have to know when it's appropriate to use a reference, pointer, or smart pointer to an object, and with C++11, you should also understand the difference between move and copy semantics.

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?

People do use reference-counting implementations. Here's an example of one:

shared_ptr<string> ref_counted = make_shared<string>("test");
shared_ptr<string> shallow_copy = ref_counted; // no deep copies, just 
                                               // increase ref count

The difference is that string doesn't do it internally as that would be inefficient for those who don't need it. Things like copy-on-write are generally not done for strings either anymore for similar reasons (plus the fact that it would generally make thread safety an issue). Yet we have all the building blocks right here to do copy-on-write if we wish to do so: we have the ability to swap strings without any deep copying, we have the ability to make pointers, references, or smart pointers to them.

To use C++ effectively, you have to get used to this way of thinking involving value semantics. If you don't, you might enjoy the added safety and convenience but do it at heavy cost to the efficiency of your code (unnecessary copies are certainly a significant part of what makes poorly written C++ code slower than C). After all, your original test is still dealing with pointers to strings, not char[] arrays. If you were using character arrays and not pointers to them, you'd likewise need to strcpy to swap them. With strings you even have a built-in swap method to do exactly what you are doing in your test efficiently, so my advice is to spend a bit more time learning C++.

Answer 6

If you have an indication of the eventual size of your vector you can prevent excessive resizes by calling reserve() before filling it up.

Answer 7

The main rules of optimization:

• Rule 1: Don't do it.
• Rule 2: (For experts only) Don't do it yet.

Are you sure that you have proven that it is really the STL that is slow, and not your algorithm?

Answer 8

Good performance isn't always easy with STL, but generally, it is designed to give you the power. I found Scott Meyers' "Effective STL" an eye-opener for understanding how to deal with the STL efficiently. Read!

As others said, you are probably running into frequent deep copies of the string, and compare that to a pointer assignment / reference counting implementation.

Generally, any class designed towards your specific needs, will beat a generic class that's designed for the general case. But learn to use the generic class well, and learn to ride the 80:20 rules, and you will be much more efficient than someone rolling everything on their own.

---

One specific drawback of std::string is that it doesn't give performance guarantees, which makes sense. As Tim Cooper mentioned, STL does not say whether a string assignment creates a deep copy. That's good for a generic class, because reference counting can become a real killer in highly concurrent applications, even though it's usually the best way for a single threaded app.

Answer 9

They didn't go wrong. STL implementation is generally speaking better than yours.

I'm sure that you can write something better for a very particular case, but a factor of 2 is too much... you really must be doing something wrong.

Answer 10

If used correctly, std::string is as efficient as char*, but with the added protection.

If you are experiencing performance problems with the STL, it's likely that you are doing something wrong.

Additionally, STL implementations are not standard across compilers. I know that SGI's STL and STLPort perform generally well.

That said, and I am being completely serious, you could be a C++ genius and have devised code that is far more sophisticated than the STL. It's not likely , but who knows, you could be the LeBron James of C++.

Answer 11

I would say that STL implementations are better than the traditional implementations. Also did you try using a list instead of a vector, because vector is efficient for some purpose and list is efficient for some other

Answer 12

std::string will always be slower than C-strings. C-strings are simply a linear array of memory. You cannot get any more efficient than that, simply as a data structure. The algorithms you use (like strcat() or strcpy()) are generally equivalent to the STL counterparts. The class instantiation and method calls will be, in relative terms, significantly slower than C-string operations (even worse if the implementation uses virtuals). The only way you could get equivalent performance is if the compiler does optimization.

Answer 13

                        string  const string&   char*   Java string
---------------------------------------------------------------------------------------------------
Efficient               no **       yes         yes     yes
assignment                          

Thread-safe             yes         yes         yes     yes

memory management       yes         no          no      yes
done for you

** There are 2 implementations of std::string: reference counting or deep-copy. Reference counting introduces performance problems in multi-threaded programs, EVEN for just reading strings, and deep-copy is obviously slower as shown above. See: Why VC++ Strings are not reference counted?

As this table shows, 'string' is better than 'char*' in some ways and worse in others, and 'const string&' is similar in properties to 'char*'. Personally I'm going to continue using 'char*' in many places. The enormous amount of copying of std::string's that happens silently, with implicit copy constructors and temporaries makes me somewhat ambivalent about std::string.

Answer 14

A large part of the reason might be the fact that reference-counting is no longer used in modern implementations of STL.

Here's the story (someone correct me if I'm wrong): in the beginning, STL implementations used reference counting, and were fast but not thread-safe - the implementors expected application programmers to insert their own locking mechanisms at higher levels, to make them thread-safe, because if locking was done at 2 levels then this would slow things down twice as much.

However, the programmers of the world were too ignorant or lazy to insert locks everywhere. For example, if a worker thread in a multi-threaded program needed to read a std::string commandline parameter, then a lock would be needed even just to read the string, otherwise crashes could ensue. (2 threads increment the reference count simultaneously on different CPU's (+1), but decrement it separately (-2), so the reference count goes down to zero, and the memory is freed.)

So implementors ditched reference counting and instead had each std::string always own its own copy of the string. More programs worked, but they were all slower.

So now, even a humble assignment of one std::string to another, (or equivalently, passing a std::string as a parameter to a function), takes about 400 machine code instructions instead of the 2 it takes to assign a char*, a slowdown of 200 times.

I tested the magnitude of the inefficiency of std::string on one major program, which had an overall slowdown of about 100% compared with null-terminated strings. I also tested raw std::string assignment using the following code, which said that std::string assignment was 100-900 times slower. (I had trouble measuring the speed of char* assignment). I also debugged into the std::string operator=() function - I ended up knee deep in the stack, about 7 layers deep, before hitting the 'memcpy()'.

I'm not sure there's any solution. Perhaps if you need your program to be fast, use plain old C++, and if you're more concerned about your own productivity, you should use Java.

#define LIMIT 800000000
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;

TfcMessage("done", 'i', "Empty loop = %1.3f s\n"
                        "char* loop = %1.3f s\n"
                        "std::string loop = %1.3f s\n\n"
                        "slowdown = %f", 
                        emptyLoop, charLoop, stl, 
                        (stl - emptyLoop) / (charLoop - emptyLoop));