Calling constructor for an array of objects in another constructor

Question

I am trying to hardcode a bunch of values in an array of objects, FloatParameterFields by calling the constructor of each object in the array. I am calling these constructors in another constructor, ParametersServerABS().

The array of objects FloatParameterFields is a private member of the ParametersServerABS class:

private:

    FloatParameterFields _floatParameters[FloatParameter::NUM_FLOAT_PARAMS];
    bool _safeMode;

In another file, I defined the enum and struct:

enum FloatParameter {
    CHIP_CLOCK_RATE_0 = 0,
    CHIP_CLOCK_RATE_1,
    CHIP_CLOCK_RATE_2,
    CHIP_CLOCK_RATE_3,
    CHIP_CLOCK_RATE_4,
    NUM_FLOAT_PARAMS
};

enum AccessModes {
    READ_ONLY = 0,
    SAFE_WRITE,
    WRITE,
    NUM_ACCESS_MODES
};

struct FloatParameterFields {

    public:

        FloatParameterFields() :
                upperBound(0),
                lowerBound(0),
                value(0),
                id(NUM_FLOAT_PARAMS),
                mode(NUM_ACCESS_MODES) {
        }

        FloatParameterFields(float upperBound, float lowerBound, float value, FloatParameter id,
                AccessModes mode) :
                upperBound(upperBound),
                lowerBound(lowerBound),
                value(value),
                id(id),
                mode(mode) {
        }

        float upperBound;
        float lowerBound;
        float value;
        FloatParameter id;
        AccessModes mode;

};

In my first attempt I did:

ParametersServerABS::ParametersServerABS() :
        _safeMode(true) {

    // Hardcoding the values to the struct
    _floatParameters[FloatParameter::CHIP_CLOCK_RATE_0](2.0, 2.0, 1.0,
            FloatParameter::CHIP_CLOCK_RATE_0, AccessModes::WRITE);
    _floatParameters[FloatParameter::CHIP_CLOCK_RATE_1](2.0, 2.0, 1.0,
            FloatParameter::CHIP_CLOCK_RATE_1, AccessModes::SAFE_WRITE);
    _floatParameters[FloatParameter::CHIP_CLOCK_RATE_2](2.0, 2.0, 1.0,
            FloatParameter::CHIP_CLOCK_RATE_2, AccessModes::WRITE);
    _floatParameters[FloatParameter::CHIP_CLOCK_RATE_3](2.0, 2.0, 1.0,
            FloatParameter::CHIP_CLOCK_RATE_3, AccessModes::WRITE);
    _floatParameters[FloatParameter::CHIP_CLOCK_RATE_4](2.0, 2.0, 1.0,
            FloatParameter::CHIP_CLOCK_RATE_4, AccessModes::READ_ONLY);

}

This gave the error:

no match for call to 'FloatParameterFields) (double, double, double, FloatParameter, AccessModes)'

Upon further research I tried:

    // Hardcoding the values to the struct
    _floatParameters[FloatParameter::NUM_FLOAT_PARAMS]= {FloatParameterFields(2.0, 2.0, 1.0,
                FloatParameter::CHIP_CLOCK_RATE_0, AccessModes::WRITE),
        FloatParameterFields(2.0, 2.0, 1.0,
                FloatParameter::CHIP_CLOCK_RATE_1, AccessModes::SAFE_WRITE),
        FloatParameterFields(2.0, 2.0, 1.0,
                FloatParameter::CHIP_CLOCK_RATE_2, AccessModes::WRITE),
        FloatParameterFields(2.0, 2.0, 1.0,
                FloatParameter::CHIP_CLOCK_RATE_3, AccessModes::WRITE),
        FloatParameterFields(2.0, 2.0, 1.0,
                FloatParameter::CHIP_CLOCK_RATE_4, AccessModes::READ_ONLY)}

And got the error:

no match for 'operator=' (operand types are 'FloatParameterFields' and '<brace-enclosed initializer list>')

I am most confused by my first attempt, why is it wrong? It seems to make sense yet I get an error... I even properly included the header files.

Joseph Mansfield · Accepted Answer · 2015-01-26 09:51:41Z

1

Before entering the opening { of your constructor, all of your members are already initialised. That includes the _floatParameters array and all of its elements. When you then do...

_floatParameters[FloatParameter::CHIP_CLOCK_RATE_0](2.0, 2.0, 1.0,
        FloatParameter::CHIP_CLOCK_RATE_0, AccessModes::WRITE);

...this is seen as attempting to call _floatParameters[FloatParameter::CHIP_CLOCK_RATE_0] as though it were a function-like object. It is not seen as an initialiser, since the objects have already been initialised.

Similarly, you later do:

_floatParameters[FloatParameter::NUM_FLOAT_PARAMS]= { ... };

which is attempting to use the initialisation syntax for an array after the array has already been initialised.

Instead, you could use the constructor's member initialisation list:

ParametersServerABS::ParametersServerABS()
  : _safeMode(true),
    _floatParameters{
      {2.0, 2.0, 1.0, FloatParameter::CHIP_CLOCK_RATE_0, AccessModes::WRITE},
      // and so on...
    }
{
}

answered Jan 26, 2015 at 9:51

Joseph Mansfield

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2 Comments

jlcv Over a year ago

Oh, were they initialized in the part where I initialized a private array of structs?

Joseph Mansfield Over a year ago

@JustinLiang They are initialised when the object is constructed, immediately before entering the body of the constructor.

polkovnikov.ph · Accepted Answer · 2015-01-26 09:52:50Z

You're calling _floatParameters[FloatParameter::CHIP_CLOCK_RATE_0](...) as if you had an array of functions, and you're calling one of its items. Even though arrays of functions do exist in C++, it doesn't seem to be what you tried to achieve.

Simply, you cannot call constructor like that. Here's a correct example:

_floatParameters[FloatParameter::CHIP_CLOCK_RATE_0] = FloatParameterFields(2.0, 2.0, 1.0, FloatParameter::CHIP_CLOCK_RATE_0, AccessModes::WRITE);

Your another approach could have worked, but there's a bug.

_floatParameters[FloatParameter::NUM_FLOAT_PARAMS]= { ... }

You're assigning an initializer list (i.e. list of values) into the FloatParameter::NUM_FLOAT_PARAMS-th item of _floatParameters. To setup an array you'd assign to _floatParameters directly. That will work with vector, but won't work with plain old arrays.

_floatParameters = { ... }

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