How to declare array in header and init in cpp

Question

I'm writing Saleae Custom Analyzer but I am also new to C++
This does not work inside of a class: Declare array in C++ header and define it in cpp file?
How to do it in a class?

class SimpleSerialSimulationDataGenerator
{
public:
    SimpleSerialSimulationDataGenerator();
    ~SimpleSerialSimulationDataGenerator();

    void Initialize( U32 simulation_sample_rate, SimpleSerialAnalyzerSettings* settings );
    U32 GenerateSimulationData( U64 newest_sample_requested, U32 sample_rate, SimulationChannelDescriptor** simulation_channel );

protected:
    SimpleSerialAnalyzerSettings* mSettings;
    U32 mSimulationSampleRateHz;

protected:
    void CreateSerialByte();
    U8 mSerialText[3] = {0xAA, 0x01, 0x55};
    U32 mStringIndex = 0;

    SimulationChannelDescriptor mSerialSimulationData;

};

mSerialText is what I would like to init below in .cpp not in header:

SimpleSerialSimulationDataGenerator::SimpleSerialSimulationDataGenerator()
{
    mSerialText = {0xAA, 0x01, 0x55};
}

but in cpp it says 'must be lvalue'
1.Can I change length later on?
2.Can I change value later on?
My dream solution is to declare array in header file with no length and init in cpp file with random data.

I get what you're getting at, @NathanOliver, but only because I understand C++, whereas Lukasz is a self-admittedly new to C++ and I think your answer is too brief. So perhaps you could actually provide a little bit of text with your answer ... and maybe even write it as an answer rather than a comment. — Wyck
– Wyck, Commented Feb 20, 2018 at 15:48
1) Did you try initializing array in constructor initializer list? Example. 2) "Can I change length later on?" Of an array? No. 3) "Can I change value later on?" Sure, you can manipulate the data in the array, in any way you want. 4) Consider learning from a good C++ book. — Algirdas Preidžius
– Algirdas Preidžius, Commented Feb 20, 2018 at 15:52
@Wyck: You are absolutely right. On the other hand, Lukasz should read his C++ book; repeating the very basics of the language 50 times a day is not what any of us is here for. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Feb 20, 2018 at 16:03

Maroš Beťko · Accepted Answer · 2018-02-20 15:52:54Z

3

For variable length array in C++ use std::vector.

In your header file you will have this:

std::vector<U8> mSerialText;

And then in source file you can initialize it with {} syntax just like in your example like this:

mSerialText = {0xAA, 0x01, 0x55};

It has an index access with operator[] or at function with out of range checks. To insert new elements, call push_back. You can read further information about how to use vector on here

edited Feb 20, 2018 at 15:52

answered Feb 20, 2018 at 15:48

Maroš Beťko

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6 Comments

Patwie Over a year ago

This just works with std=c++11, otherwise extended initializer lists only available with -std=c++11 or -std=gnu++11

Maroš Beťko Over a year ago

He haven't said anywhere that he needs to use old version of c++.

Patwie Over a year ago

But it is tagged with C++ only. Anyway, this answer is the way to go! Otherwise, the answer would be use c++11 and then follow this answer.

Maroš Beťko Over a year ago

Since he said he is new to C++, I don't think he even knows what c++11 means. Also, c++11 or even c++14 is nowadays used by default if you don't change compile settings in f.e. visual studio.

Lightness Races in Orbit Over a year ago

@MarošBeťko: That's pretty short-sighted; there are plenty of OSs out there on which e.g. GCC 4.4 is still the default, which doesn't even have C++11 or C++14. If the OP isn't aware of these distinctions, even more reason to explain them.

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