Creating object pointers in C++

Question

I'm trying to create an object which I want to populate inside a switch-case, but it's out of my knowledge scope.

I have these constructors:

cObj::cObj()
{
}

cObj::cObj(std::string filename)
{
    //...
}

So, basically I want to call following method, create a pointer to the object, and populate it inside of my switch-case:

void someThing() {
    cObj myObj();

    switch (someValue)
        case 0:
            myObj("/some/path");
            break;
        ...
}

I assume my constructor is wrong since it does not really work.

How can I accomplish this?

Benjamin Lindley · Accepted Answer · 2012-05-25 18:57:04Z

4

You don't need a pointer for this.

void someThing() {
    cObj myObj; // Don't use parentheses for the default constructor.
                // What you had was a function declaration, not an object creation.

    switch (someValue) {
        case 0:
            myObj = cObj("/some/path");
            break;
        ...
}

If you didn't have a default constructor, or you didn't want it to be called, then you could use a pointer, preferably smart:

void someThing() {
    std::unique_ptr<cObj> myObj;

    switch (someValue) {
        case 0:
            myObj.reset(new cObj("/some/path"));
            break;
        ...
}

Or, as lmmilewski hinted at, you could factor out the decision to a function, and return the object:

cObj choose(someType someValue) {
    switch (someValue) {
        case 0:
            return cObj("/some/path");
        ...
}

void someThing() {
    cObj myObj(choose(someValue));
    ...
}

edited May 25, 2012 at 18:57

answered May 25, 2012 at 18:41

Benjamin Lindley

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2 Comments

JavaCake Over a year ago

Its as if when i try to use the object in the first method you mentioned its empty? The constructor is called, but when i use the object its empty..

Benjamin Lindley Over a year ago

@JavaCake: It's default constructed. Then in the switch statement, I assign it a new value. (assuming you haven't done something to disable the assignment operator)

Łukasz Milewski · Accepted Answer · 2012-05-25 18:46:21Z

3

When you create your object in this line

cObj myObj();

(btw. you probably don't want these parentheses. You want to create an object, not declare a function).

you call the constructor. You can not call it again in switch statement. You could create a separate method:

cObj::cObj()
{
}

void cObj::SetFilename(const std::string& filename) {
// ...
}

and use it like that:

void someThing() {
    cObj myObj;

    switch (someValue)
        case 0:
            myObj.SetFilename("/some/path");
            break;
        ...
}

I'm not sure what you're trying to do, but maybe better way would be to first determine what the filepath is and then create the object?

void someThing() {
    std::string filepath = "default/path";

    switch (someValue)
        case 0:
            filepath = "some/path";
            break;
        ...

    cObj myObj(flepath);
}

You could also create a function that would make the decision what path to use and return the object:

answered May 25, 2012 at 18:46

Łukasz Milewski

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3 Comments

Cat Plus Plus Over a year ago

cObj myObj(); doesn't create the object, because of most vexing parse — it's actually a function declaration.

Benjamin Lindley Over a year ago

+1 for your last idea (factoring out the decision to a function), which is what I normally do in situations like this.

JavaCake Over a year ago

The last method is very clean i like that.

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