CODEWITHSUNDEEP
c++constructortemporary-objects
larrylampco
larrylampco

Reputation: 597

Passing a Constructor to a Function

I am using an STL vector that is a vector of Parameters.

std::vector<Parameter> foo;

I was trying to find a way to add Parameter objects to the vector without doing this:

Parameter a;
foo.push_back(a);

I came across an implementation that did this:

foo.push_back(Parameter()); //Using the Parameter constructor

I thought that when I created an object the constructor is called not vise versa. Why can I pass a constructor to a function?

Upvotes: 7

Views: 11222

Answers (6)

Fayeure
Fayeure

Reputation: 1369

This subject is old but I'd like to add something, if you truly want to pass a constructor as parameter (as function pointer) without creating a temporary object you can do this hack if you're using a recent enough C++ standard

foo([](){ return MyClass(); });

Upvotes: 0

Simon Bosley
Simon Bosley

Reputation: 1154

The other point we're missing here is that to pass a function as a parameter you do not use parentheses at all. Here's an example of passing a function as a parameter: 

#include <algorithm>
#include <vector>
#include <iostream>

// initialise sum and summing function
int sum = 0;
void sum_numbers( const int& number );

// Create a test for our sum function
int main(int argc, const char * argv[])
{
    // create some numbers
    std::vector<int> numbers;
    numbers.push_back( 34 );
    numbers.push_back( 73 );
    numbers.push_back( 14 );

    // passing "sum_numbers" function as a parameter
    std::for_each( numbers.begin(), numbers.end(), sum_numbers );

    // check that our sum function worked
    std::cout << "The sum of numbers is: " << sum << std::endl;

    return 0;
}

// add the current number to the sum
void sum_numbers( const int& number )
{
    sum += number;
}

Upvotes: 0

legends2k
legends2k

Reputation: 32894

foo.push_back(Parameter()); is passing a temporarily constructed Parameter object to push_back and not the constructor i.e. Parameter() is a call to create an object of type Parameter on the stack and pass it to the push_back function of vector, where it gets moved/copied. Thus what gets passed is not the constructor itself, but a constructed object only.

It's just a shorthand way of writing Parameter a; foo.push_back(a); when one is sure that a is not used anywhere down the line; instead of declaring a dummy, temporary variable, an anonymous temporary is created and passed.

These might be useful if you want to learn more about temporaries:

http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=198

http://msdn.microsoft.com/en-us/library/a8kfxa78%28v=vs.80%29.aspx

Upvotes: 8

Priyatham51
Priyatham51

Reputation: 1884

when you call Parameter() a temporary object is created and passed to foo.push_back() function. Previously you declared the object with name Parameter a; and passed it to like this foo.push_back(a). By doing like this you can use the object named a down the line of your program.

Upvotes: 1

dchhetri
dchhetri

Reputation: 7136

You are not passing a constructor, but instead passing a temporary object. When you do Paramater() because of the pharenthesis, it creates a object. Its kind of like a function call for example getInput().

Upvotes: 0

David G
David G

Reputation: 96800

That line will create a temporary instance of Parameter and copy it into Foo. Assuming this is pre-C++11 code. The new std::vector<T>::push_back has an overload for rvalues in which case there will be no copies.

Upvotes: 4

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