C++ string vs C# string, different running times. Why?

Question

I was experimenting with C++ to observe the effects of variables' scope-bounded-declarations and usage in loops on the running time of the program, as follows:

for(int i=0; i<10000000 ; ++i){
    string s = "HELLO THERE!";
}

and

string s;
for(int i=0; i<10000000 ; ++i){
    s = "HELLO THERE!";
}

The first program run in ~1 second while the second one run in ~250 milliseconds, as expected. Trying built in types wouldn't cause a significant difference, so I stick with strings in both languages.

I was discussing this with a friend of mine and he said this wouldn't happen in C#. We tried and observed ourselves that this did not happen in C# as it turned out, scope-bounded declarations of strings won't affect running time of the program.

Why is this difference? Is that a bad optimization in C++ strings (I strongly doubt that tho) or something else?

I'm fairly sure you did this test without optimizations enabled in C++. Most C++ compilers will get rid of that entire chunk of code because it has no observable side effects. — Ben Voigt
– Ben Voigt, Commented Apr 26, 2014 at 19:34

Thomas · Accepted Answer · 2014-04-26 19:30:36Z

7

Strings in C# are immutable, so the assignment can just copy over a reference. In C++, however, strings are mutable, so the entire contents of the string need to be copied over.

If you want to verify this hypothesis, try with a (significantly) longer string constant. Runtime in C++ should go up, but runtime in C# should remain the same.

answered Apr 26, 2014 at 19:30

Thomas

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1 Comment

Ben Voigt Over a year ago

Right, the equivalent in C++ would be to use const char* s;

user3080385 · Accepted Answer · 2014-04-26 20:41:27Z

1

Strings in C# are immutable. C# uses references and the memory it's not copied!

in C# "HELLO THERE!" will be automatically assigned to a piece of memory and won't be copied each time for example:

string a = "HELLO"; string b = a;

they are pointing to the same piece of memory, but in C++ no! the string will be the same but not in the same place, if you want to obtain the same reult you should use pointers (or smart pointers)

string *a = new string("hello"); string *b = a;

answered Apr 26, 2014 at 20:41

user3080385

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