Iterators and functors

[tryptik]

Hello all,

I have a question about iterators. I have a container of functors
that operate on an std::string. The functors go something like this:

class Functor {
std::string operator()(const std::string& s) {/*manipulate
string*/; return newString;}
};

Now, I want to call the functors on a string argument. I write
something like this:

std::vector<Functor> vec;

/*Omitted creation and push_back of a couple Functors */

std::vector<Functor>::const_iterator it = vec.begin();
std::vector<Functor>::const_iterator end = vec.end();

std::string arg("Test argument");

for(; it != end; it++)
std::cout << *it(arg) << "\n"; //Why doesn't this work?

Instead of using the de-reference operator '*', I have to write the
following:

it->operator()(arg);

Can some kind soul explain to me why this is? I prefer the cleaner
syntax of the first statement.

-tryptik

 

[taylor.hugg]

Hello all,

I have a question about iterators. I have a container of functors
that operate on an std::string. The functors go something like this:

class Functor {
std::string operator()(const std::string& s) {/*manipulate
string*/; return newString;}

};

Now, I want to call the functors on a string argument. I write
something like this:

std::vector<Functor> vec;

/*Omitted creation and push_back of a couple Functors */

std::vector<Functor>::const_iterator it = vec.begin();
std::vector<Functor>::const_iterator end = vec.end();

std::string arg("Test argument");

for(; it != end; it++)
std::cout << *it(arg) << "\n"; //Why doesn't this work?

Instead of using the de-reference operator '*', I have to write the
following:

it->operator()(arg);

Can some kind soul explain to me why this is? I prefer the cleaner
syntax of the first statement.

-tryptik

well that is a hard question to anser but i think it is wrong

 

[John Harrison]

Hello all,

I have a question about iterators. I have a container of functors
that operate on an std::string. The functors go something like this:

class Functor {
std::string operator()(const std::string& s) {/*manipulate
string*/; return newString;}

Functors should be const

std::string operator()(const std::string& s) const
{
...
}

};

Now, I want to call the functors on a string argument. I write
something like this:

std::vector<Functor> vec;

/*Omitted creation and push_back of a couple Functors */

std::vector<Functor>::const_iterator it = vec.begin();
std::vector<Functor>::const_iterator end = vec.end();

std::string arg("Test argument");

for(; it != end; it++)
std::cout << *it(arg) << "\n"; //Why doesn't this work?

Instead of using the de-reference operator '*', I have to write the
following:

it->operator()(arg);

Can some kind soul explain to me why this is? I prefer the cleaner
syntax of the first statement.

-tryptik

Just a question of operator precedence I think, try this.

std::cout << (*it)(arg) << "\n";

john

 

[Victor Bazarov]

I have a question about iterators. I have a container of functors
that operate on an std::string. The functors go something like this:

class Functor {
std::string operator()(const std::string& s) {/*manipulate
string*/; return newString;}
};

Now, I want to call the functors on a string argument. I write
something like this:

std::vector<Functor> vec;

/*Omitted creation and push_back of a couple Functors */

std::vector<Functor>::const_iterator it = vec.begin();
std::vector<Functor>::const_iterator end = vec.end();

std::string arg("Test argument");

for(; it != end; it++)
std::cout << *it(arg) << "\n"; //Why doesn't this work?

What do you mean by "doesn't work"? Does it compile?

Instead of using the de-reference operator '*', I have to write the
following:

it->operator()(arg);

Can some kind soul explain to me why this is? I prefer the cleaner
syntax of the first statement.

Use

(*it)(arg);

And read your favourite C++ book again about the precedence of operators.

V

 

[Jim Langston]

Hello all,

I have a question about iterators. I have a container of functors
that operate on an std::string. The functors go something like this:

class Functor {
std::string operator()(const std::string& s) {/*manipulate
string*/; return newString;}
};

Now, I want to call the functors on a string argument. I write
something like this:

std::vector<Functor> vec;

/*Omitted creation and push_back of a couple Functors */

std::vector<Functor>::const_iterator it = vec.begin();
std::vector<Functor>::const_iterator end = vec.end();

std::string arg("Test argument");

for(; it != end; it++)
std::cout << *it(arg) << "\n"; //Why doesn't this work?

As stated by others, this is actually:
*(it(arg))
But what you really want is
(*it)(arg)
so you have to specify that.

I now always use (*it) when derefernecing iterators for any context.