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C++20 features: constexpr virtual functions, explicit(bool), char8_t.

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Anthony Calandra
2020-02-01 17:59:06 -05:00
parent 71c24af74f
commit 6d90199ea9
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46
CPP20.md
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@@ -11,6 +11,9 @@ C++20 includes the following new language features:
- [likely and unlikely attributes](#likely-and-unlikely-attributes)
- [deprecate implicit capture of this](#deprecate-implicit-capture-of-this)
- [class types in non-type template parameters](#class-types-in-non-type-template-parameters)
- [constexpr virtual functions](#constexpr-virtual-functions)
- [explicit(bool)](#explicit-bool)
- [char8_t](#char8_t)
C++20 includes the following new library features:
- [concepts library](#concepts-library)
@@ -245,6 +248,49 @@ get_foo(); // uses implicit constructor
get_foo<foo{123}>();
```
### constexpr virtual functions
Virtual functions can now be `constexpr` and evaluated at compile-time. `constexpr` virtual functions can override non-`constexpr` virtual functions and vice-versa.
```c++
struct X1 {
virtual int f() const = 0;
};
struct X2: public X1 {
constexpr virtual int f() const { return 2; }
};
struct X3: public X2 {
virtual int f() const { return 3; }
};
struct X4: public X3 {
constexpr virtual int f() const { return 4; }
};
constexpr X4 x4;
x4.f(); // == 4
```
### explicit(bool)
Conditionally select at compile-time whether a constructor is made explicit or not. `explicit(true)` is the same as specifying `explicit`.
```c++
struct foo {
// Specify non-integral types (strings, floats, etc.) require explicit construction.
template <typename T>
explicit(!std::is_integral_v<T>) foo(T) {}
};
foo a = 123; // OK
foo b = "123"; // ERROR: explicit constructor is not a candidate (explicit specifier evaluates to true)
foo c {"123"}; // OK
```
### char8_t
Provides a standard type for representing UTF-8 strings.
```c++
char8_t utf8_str[] = u8"\u0123";
```
## C++20 Library Features
### Concepts library

46
README.md
View File

@@ -11,6 +11,9 @@ C++20 includes the following new language features:
- [likely and unlikely attributes](#likely-and-unlikely-attributes)
- [deprecate implicit capture of this](#deprecate-implicit-capture-of-this)
- [class types in non-type template parameters](#class-types-in-non-type-template-parameters)
- [constexpr virtual functions](#constexpr-virtual-functions)
- [explicit(bool)](#explicit-bool)
- [char8_t](#char8_t)
C++20 includes the following new library features:
- [concepts library](#concepts-library)
@@ -337,6 +340,49 @@ get_foo(); // uses implicit constructor
get_foo<foo{123}>();
```
### constexpr virtual functions
Virtual functions can now be `constexpr` and evaluated at compile-time. `constexpr` virtual functions can override non-`constexpr` virtual functions and vice-versa.
```c++
struct X1 {
virtual int f() const = 0;
};
struct X2: public X1 {
constexpr virtual int f() const { return 2; }
};
struct X3: public X2 {
virtual int f() const { return 3; }
};
struct X4: public X3 {
constexpr virtual int f() const { return 4; }
};
constexpr X4 x4;
x4.f(); // == 4
```
### explicit(bool)
Conditionally select at compile-time whether a constructor is made explicit or not. `explicit(true)` is the same as specifying `explicit`.
```c++
struct foo {
// Specify non-integral types (strings, floats, etc.) require explicit construction.
template <typename T>
explicit(!std::is_integral_v<T>) foo(T) {}
};
foo a = 123; // OK
foo b = "123"; // ERROR: explicit constructor is not a candidate (explicit specifier evaluates to true)
foo c {"123"}; // OK
```
### char8_t
Provides a standard type for representing UTF-8 strings.
```c++
char8_t utf8_str[] = u8"\u0123";
```
## C++20 Library Features
### Concepts library