QSettings::setValue() replacing custom type with invalid type?

[CJha]

Hi, in my application I am storing the settings using settings.ini file format. I have a custom data type that I register and then retrieve and store in the settings.ini file. During the startup, before the custom data type is registered using qRegisterMetaType, I call QSettings::setValue() to set a normal integer value (completely independent from the custom data type) in the settings.ini file. But doing this makes the QSettings replace all the custom types currently in the settings.ini file with an invalid QVariant type while debugging the output QVariant::load: unknown user type with name CustomDataType, is this behavior normal?

[jsulm]

@CJha The problem is that you can't just replace something in a file if new value has different lenght than the old one. In this case you need to read the file into memory, replace the value and write it back. To read such an ini file and replace something there Qt needs to know how to interpret the data it reads.

[jsulm]

@CJha said in QSettings::setValue() replacing custom type with invalid type?:

is this behavior normal?

I think so, because at that time you did not yet registered you custom type, right?

[JonB]

@CJha
Thinking aloud. In order to do any setValue() (or just value(), or any other QSettings operation) the Qt code is going to have to read the whole of the .ini file and parse it correctly. It cannot just "access a single key-value". So it seems reasonable to me that it will fall over if the file contains any unknown custom data type at that point. You really need to call any qRegisterMetaType() before you try to access anything in the .ini file, ISTM.

[CJha]

@jsulm Yes, the custom data type is registered afterward when the code execution reaches that point.

[CJha]

@JonB Thanks! I was thinking the same, but it is kind of a pity that a single key-value pair cannot be accessed without checking each entry in the .ini file, after all keys are stored as text and should be easy to filter out.

[jsulm]

@CJha The problem is that you can't just replace something in a file if new value has different lenght than the old one. In this case you need to read the file into memory, replace the value and write it back. To read such an ini file and replace something there Qt needs to know how to interpret the data it reads.