不懂 JLospinoso 书籍的模板实现

Question

In the JLospinoso (c++ Crash Course), chapter 10 about unit testing. Link to the book page

There is this code (simplified)

struct BrakeCommand {
    double time_to_collision_s;
};

template<typename T>
struct AutoBrake {
    AutoBrake(const T& publish) : publish{publish} {}

---snips---

private:
    const T& publish;
}

Which is called

AutoBrake auto_brake{[](const BrakeCommand&) {}};

1 - I absolutely don't understand why there is this much {} []?? And nothing about the template...

2 - C++ compiler MSVC raise this error

"error C2955: 'AutoBrake': use of class template requires template argument list"

Why is there a template here? How to compile this program?

Thanks a lot,

Answer 1

set(CMAKE_CXX_STANDARD 17)

And everything works fine !

Answer 2

{}[] symbols which are confused you it's a part of the lambda definition passed by initializer_list to AutoBrake constructor.

Let's go through the code line step by step

AutoBrake auto_brake{[](const BrakeCommand&) {}};

1 Lambda which takes BrakeCommand as an argument and does nothing

[](const BrakeCommand&) {};

2 The constructor with initialized_list, lambda as an argument for it

auto variable = [](const BrakeCommand&) {}; // to simplify understanding what is going on further
AutoBrake auto_brake{variable}; // it might be replaced by AutoBrake auto_brake(variable);

Now let's figure out how the template works.

AutoBrake auto_brake{[](const BrakeCommand&) {}};

By this was declared that the template parameter T, for your instance is a function.

So code generated by preprocessor from your template will look as

template<std::function>
struct AutoBrake {
    AutoBrake(const std::function& publish) : publish{publish} {}

---snips---

private:
    const std::function& publish;
}