trainings/AdvancedCppV2/Presentation/moder_cpp_cpp17_small_features.md

## Nested namespace definitions
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You can nest namespaces like this:
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```c++
namespace A::B::C {
  ...
}
```
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Instead of this:
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```c++
namespace A {
  namespace B {
    namespace C {
      ...
    }
  }
}
```
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## Class template argument deduction

From C++17 class template arguments can be deduced automatically. Automatic template argument deduction was available earlier only for template functions.

```c++
std::pair p(1, 'x');                    // C++17: OK, C++14: error: missing.
                                        // std::pair<int, char> 
std::pair<int, std::string> p(1, "x");  // C++14: OK
                                        // std::pair<int, std::string> 
auto p2 = std::make_pair(1, "x");       // C++17: OK, C++14: OK (but not string!)
                                        // std::pair<int, const char*> 
std::pair p3(1, "x");                   // C++17: OK (but not string!), C++14 error
```

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## Selection statements with initializer (1)

New versions of the `if` and `switch` statements for C++:
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### `if (init; condition)`
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```cpp
status_code foo() { // C++14
    { //variable c scope
        status_code c = bar();
        if (c != SUCCESS) {
            return c;
        }
    }
    // ...
}
```
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```cpp
status_code foo() { // C++17
    if (status_code c = bar(); c != SUCCESS) {
        return c;
    }
    // ...
}
```
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## Selection statements with initializer (2)

### `switch (init; condition)`
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```C++
class Foo {
public:
    enum class ErrorCode {
        Ok,
        Bad,
        VeryBad
    };

    ErrorCode doSth() {}
}

int main() {
    switch (Foo foo ; const auto err = foo.doSth()) {
    case ErrorCode::Ok:
        break;
    case ErrorCode::Bad:
        break;
    case ErrorCode::VeryBad:
        break;
    }
}
```
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## Selection statements with initializer (3)

```C++
class ThreadSafeQueue {
public:
    std::optional<int> try_pop() {
        int val = 0;
        // Lock guard is visible inside if-else
        if (std::lock_guard lock(m_) ; queue_.empty()) {
            return {};
        } else {
            val = queue_.front();
            queue_.pop();
        }
        
        // Here mutex is no longer locked
        logger << "pop value from queue";
        return val;
    }

private:
    std::queue<int> queue_;
    std::mutex m_;
};
```
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## Hiding variable inside <code>if-else</code> statement (2)

We can avoid assigning iterators and then comapre it in `if` statement. Now we can write it faster

```C++
int main() {
    std::vector<int> vec;

    if (const auto it = std::find(std::cbegin(vec), std::cend(vec), 10) ; it != std::cend(vec)) {
        // *it* is visible here
    } else {
        // and here
    }
    // but not here

    std::map<std::string, int>
    if (const auto it = map.find("Ala") ; it != std::cend(map)) {
        // ...
    } else {
        // ...
    }
}
```
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## Unified initialization

* <!-- .element: class="fragment fade-in" --> C++11 introduce <code>{}</code> for initialization
* <!-- .element: class="fragment fade-in" --> C++17 fixed some problems with <code>{}</code> and unified it.

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## Unified initialization C++11

* Guess output (compiler 4.7.4 with C++11 flag)

<pre><code class="cpp" data-trim data-line-numbers data-noescape>
auto a {1};             <span class="fragment">//std::initializer_list&lt;int></span>
auto b = {1};           <span class="fragment">//std::initializer_list&lt;int></span>
auto c {1, 2};          <span class="fragment">//std::initializer_list&lt;int></span>
auto d = {1, 2};        <span class="fragment">//std::initializer_list&lt;int></span>
auto e = {1, 2, 3.f};   <span class="fragment">//Not compile               </span>
auto f {1, 2, 3.f}      <span class="fragment">//Not compile               </span>
</code></pre>

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## Unified initialization C++17

* Guess output (compiler 8.4 with C++17 flag)

<pre><code class="cpp" data-trim data-line-numbers data-noescape>
auto a {1};             <span class="fragment">//int                       </span>
auto b = {1};           <span class="fragment">//std::initializer_list&lt;int></span>
auto c {1, 2};          <span class="fragment">//Not compile (mising =)              </span>
auto d = {1, 2};        <span class="fragment">//std::initializer_list&lt;int></span>
auto e = {1, 2, 3.f};   <span class="fragment">//Not compile               </span>
auto f {1, 2, 3.f}      <span class="fragment">//Not compile               </span>
</code></pre>

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## Structural biding

* <!-- .element: class="fragment fade-in" --> Unpack strucures, classes, tuples, pairs etc...

```C++
struct Foo{};

std::tuple<int, std::string, Foo> getTuple() {
    return {5, "Ala has a cat", Foo{}};
}

struct Bar {
    std::string str_;
    double val_;
    char c_;
    std::vector<int> vec_;
};

int main() {
    const auto& [id, topic, foo] = getTuple();

    std::vector<Bar> bar;
    for (const auto& [name, value, sign, vec] : bar) {
        // ...
    }

    std::map<int, std::string> map;
    for (const auto& [key, value] : map) {
        // ..
    }
};
```
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