cpp-tutor | Code examples for tutoring modern C | Machine Learning library

The way the compilers I am familiar with implement closures is to effectively create a struct of all the captures and pass a pointer to the struct as a parameter to the function. The compiler will only add to the struct those captures explicitly listed, and in the case of the general capture, only those visible variables that are actually used in the body of the lambda.

So in short, explicitly listing the captures for a lambda is possibly a pessimization since it might copy more values than are strictly necessary. The optimizer can and does eliminate some unused captures or change them from by-reference to by-value when it can do so.

By the time the optimizer has done its job you're unlikely to notice any performance penalty even in tight inner loops. You are better off making your captures explicit because it expresses intent and that's the expensive part of software development.

You're getting this error because Melodic is the ros distro for Ubuntu 18.04. As of present the most recent release is Noetic which targets 20.04. The version of Ubuntu you're using does not currently have a supported ROS release, as such your only real option is to downgrade if you want ROS.

You will take a copy of Complex object on those functions:

The very link you cited explains what's happening:

https://www.learncpp.com/cpp-tutorial/stdcin-and-handling-invalid-input/

When the user enters input in response to an extraction operation, that data is placed in a buffer inside of std::cin.

When the extraction operator is used, the following procedure happens:

• If there is data already in the input buffer, that data is used for extraction.
• If the input buffer contains no data, the user is asked to input data for extraction (this is the case most of the time). When the user hits enter, a ‘\n’ character will be placed in the input buffer.
• operator>> extracts as much data from the input buffer as it can into the variable (ignoring any leading whitespace characters, such as spaces, tabs, or ‘\n’).
• Any data that can not be extracted is left in the input buffer for the next extraction.

So far, so good. The article continues:

[Upon an input error] std::cin goes immediately into “failure mode”, but also assigns the closest in-range value to the variable. Consequently, x is left with the assigned value of 32767.

Additional inputs are skipped, leaving y with the initialized value of 0.

This explains the "0" you're seeing. It also explains why "z" wasn't replaced with "a".

It stores it inside the object itself. Another way to think of your lambda is below. This is "kind of" equivalent to what the compiler is generating, and yes, I'm changing scopes around a bit and I know that, but this may be clearer to a beginner at C++.

All QObjects will delete their own child objects automatically. (See docs here.) QWidgets are QObjects. So as long as you establish a parent/child relationship, you do not need to manually delete your objects. To do that, simply pass a pointer to the parent object to the constructor:

(i.e. std::string

No, string literals are not related to the class std::string. Technically this could happen if vector had a special deduction guide for this, but it doesn't.

... char*

No, string literals are const.

The contained type will be deduced as const char*.

Dynamic arrays are dynamic i.e. they have dynamic lifetime / dynamic storage (i.e. they are stored in free store aka "heap").

Dynamic arrays are also dynamic in the sense that unlike array variables, their size can be determined at runtime i.e. it doesn't need to be compile time constant. Example:

Is function template specialization really allowed?

Yes, but not partial specialisation.

It seems that function partcial template specialization is not allowed.Is it really correct?

If you mean partial specialisation, that is indeed not allowed for function templates.