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Default values for STL container function arguments?

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Join Date: Feb 2005

Posts: 466

Reputation: winbatch is on a distinguished road

Solved Threads: 18

winbatch

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Posting Pro in Training

Default values for STL container function arguments?

Nov 7th, 2005

Quick question:

Normally when I have a function, I can make one or more of the 'last' arguments as defaults, for example:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
string whatever( int a, string b="NONE" )
{
//blah, blah, blah
string temp= "WHATEVER";
return temp;
}
string whatever( int a, string b="NONE" )
{
//blah, blah, blah
string temp= "WHATEVER";
return temp;
}

The benefit is that I can have one function and I can do logic in there based on whether my parameter has the default or has something else.

I would like to do the same thing, but with a parameter that is a container. How do I do it?

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 C++ Syntax (Toggle Plain Text) 
string whateverTwo( int a, vector<string> defaultV = ????)
{
 
//whatever
string temp= "WHATEVER";
return temp;
}
string whateverTwo( int a, vector<string> defaultV = ????)
{
 
//whatever
string temp= "WHATEVER";
return temp;
}

I want this so that I can either pass in the vector or not. I know that I of course can make 2 different functions, but wanted to see if I could do a default instead. Any thoughts?

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Join Date: Aug 2005

Posts: 80

Reputation: Daishi is an unknown quantity at this point

Solved Threads: 2

Daishi

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Re: Default values for STL container function arguments?

Nov 7th, 2005

Why not use a constant pointer to the vector? If the vector is not zero, then make a copy of it inside the function.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
string whateverTwo( int a, const vector<string> *defaultV = 0) {
    vector<string> stringVector;
    if (defaultV != 0)
        stringVector = *defaultV;
    // ...
}
string whateverTwo( int a, const vector<string> *defaultV = 0) {
    vector<string> stringVector;
    if (defaultV != 0)
        stringVector = *defaultV;
    // ...
}

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Join Date: Jul 2005

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Reputation: Lerner is a name known to all

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Lerner

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Posting Virtuoso

Re: Default values for STL container function arguments?

Nov 7th, 2005

STL vector objects have their own default capacity, which is implementation dependent. You should be able to determine this value rather simply. Something like this should do.

vector<int> v;
cout << "default capacity of vectors in my STL implementation is " << v.capacity <<

Of course you can specify a capacity, if you desire.

vector<int> v(5);
cout << "the capacity of v is " << v.capacity();
cout << "the size of v is ";
if(v.empty())
cout << "zero" << endl;
else
cout << v.size();

You can also specify default values.

vector<int> v(5,0);
for(int i = 0; i < 5; ++i)
cout << "v[" << i << "] = " << v[i] << ' ' ;

Data structures can be very big, meaning passing them back and forth by value can be costly. It's often wiser to pass them by reference, making them const references if you don't want the function to change any of the values in the structure being passed. So I'd declare the structure using my own defualt capacity and default all values to the desired default value and then pass that structure to a function by reference in it's current state.