are std::strings magic?

[bigearsbilly]

I was experimenting tonight with containers.
simply reading word by word a copy of don quixote into a container,
e.g: vector<string> and then dumping it all out again.

so i thought maybe if I pass the string as a pointer, then,
as it was not being copied many times it would naturally speed it up.

strangely it was slower!

on a set<string> the book took 2 seconds with string and 5 seconds
with string*

surprising?

[Galik]

std::string uses shared references to the actual character data. So when you do a string copy you don't actually copy all the data, you merely copy a pointer to the data.

I'm not sure why a pointer would be slower than a string (seeing as strings contain pointers) but I am also not surprised that strings are very quick to copy.

[bigearsbilly]

ah yes I was using a smart pointer as recomended.

just tried again, the times are closer again.
It seems set goes odd
when using boost::shared_ptr
i lost loads of data.

need to find out why.

still it's cool.
no need to worry about slinging strings about.

[jsmith]

If you are putting pointers in the container, it means you are doing more memory allocations.

words.push_back( new std::string( w ) );

= 3 allocations:
1 for allocating a new node in set's internal data structure;
1 for the new std::string;
1 somewhere for std::string allocating it's internal buffer
(assuming there is only one; depends on your code)

words.push_back( std::string( w ) );

= 2 allocations:
1 for allocating a new node in set's internal data structure;
1 somewhere for std::string allocating it's internal buffer
(assuming there is only one; depends on your code)

[ankitsingh12]

@jsmith:
the first example is not putting pointer in the container...it is putting a string object...
similarly the second one is also putting the string object...

As far as i think putting pointer inside a container shall follow the syntax:

std::string* str = new std::string("Hello");
std::vector<std::string*> vecPstr;
vecPstr.push_back(str);

[Albatross]

@ankitsingh12:
new std::string(w) returns a pointer to a string. You should know that; you used new in your example.

That said, I hope words in that first example takes std::string*s.

-Albatross

[bigearsbilly]

conclusion:

don't worry about run time speed when slinging strings about.
just use 'em.

[Disch]

eehhhhh

I'm not sure that's good advice.

[helios]

1 for allocating a new node in set's internal data structure;
1 for the new std::string;
1 somewhere for std::string allocating it's internal buffer

Isn't the second step dwarfed in length by the other two steps? Particularly if the string is long and/or the set is big.