Can't initialize valarray as private member of class

Question

I'm trying to implement a class that contains a valarray and 2 ints that define its size. My hpp file looks something like this:

class Matrix
{
public:
    // Constructors
    Matrix();
    Matrix(int width, int height);

    // Mutators
    void setWidth(int width);               // POST: width of the matrix is set
    void setHeight(int height);             // POST: height of the matrix is set
    //void initVA(double width);

    // Accessors
    int getWidth();                         // POST: Returns the number of columns in the matrix
    int getHeight();                        // POST: Returns the number of rows in the matrix

    // Other Methods
    //void printMatrix(const char* lbl, const std::valarray<double>& a);

private:

    int width_;
    int height_;
    std::valarray<double> storage_;
};

However, when I try to initialize the valarray on the constructor like this:

Matrix::Matrix(int width, int height)
{
    width_ = width;
    height_ = height;
    storage_(width*height);
}

I keep getting this error message:

error C2064: term does not evaluate to a function taking 1 arguments

The documentation says that I can declare a valarray in at least 5 different ways, but only the default contructor works. I've looked everywhere but haven't been able to find any useful info. Any help would be appreciated.

Use the initializer list. What you're trying to do isn't valid because a) invalid syntax b) the member is already initialized at that point. — Luchian Grigore, May 08 '13 at 16:22
@LuchianGrigore The syntax is actually valid, it just refers to an `operator()` overload that doesn't exist. (And that probably wouldn't do what the OP is wanting if it did exist.) — cdhowie, May 08 '13 at 16:26
@cdhowie you're right, I was referring to invalid initialization syntax. I can see how I was a bit too brief. — Luchian Grigore, May 08 '13 at 16:35

score 3 · Accepted Answer · answered May 08 '13 at 16:22

3

You are actually trying to invoke std::valarray<double>::operator()(int) here, but no such operator exists. Presumably, you meant to use an initializer list instead:

Matrix::Matrix(int width, int height)
    : width_(width),
      height_(height),
      storage_(width*height)
{
}

Alternatively, you can assign a new object instance instead, but this will be less performant than using an initializer list, as storage_ will be default-constructed, and then replaced with a copy of the new temporary, and finally the temporary will be destructed. (A decent compiler may be able to eliminate some of these steps, but I would not rely on that.)

storage_ = std::valarray<double>(width*height);

answered May 08 '13 at 16:22

cdhowie

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That solved the problem! However, I fail to see the difference. It's been a while since I've used c++ and I've been running into multiple problems. Why does the init. list invoke the "()" operator but not the code that I initally had? – Jota May 08 '13 at 16:26
@Oniros The initializer list replaces any default-initialization of object members performed by the compiler. The syntax required for initialization in this list and for assignment outside of the list are entirely different. – cdhowie May 08 '13 at 16:27
1

@Oniros The code that you initially had *did* try to invoke `operator()`, you have that backwards. The initializer list syntax mimics construction syntax for variables (e.g. `std::valarray foo(5);`). `a(b)` has different meaning in the initializer list and outside of it. In the initializer it means "initialize `a` to `b`", while outside it means "invoke `operator()` on `a`, passing `b`." – cdhowie May 08 '13 at 16:29

Can't initialize valarray as private member of class

1 Answers1