cpp17
C++ 17 引入的新特性
- 嵌套命名空间； Nested Namespaces
- If 和 switch 中的变量声明；Variable declaration in if and switch；
- If constexpr 语句
- 结构化绑定
- 折叠表达式
- 枚举的直接列表初始化
- 内联变量
constexpr lambda表达式- 新增三个
attributes
1. Nested Namespaces 嵌套命名空间
C++17 之前
namespace Game {
namespace Graphics {
namespace Physics {
class 2D {
..........
};
} // Physics
} // Graphics
} // Game
C++17 之后
namespace Game::Graphics::Physics {
class 2D {
..........
};
}
2. Variable declaration in if and switch
C++17 之前
std::vector<std::string> str;
const auto it = std::find(std::begin(str), std::end(str), "abc");
if (it != std::end(str))
{
*it = "$$$";
}
C++17 之后
std::vector<std::string> str;
if (const auto it = std::find(std::begin(str), std::end(str), "abc"); it != std::end(str) )
{
*it = "$$$";
}
3. If constexpr statement
if constexpr is evaluated at compile time, like constexpr function.
template <typename T>
auto length(T const &value)
{
if constexpr(is_integral<T>::value)
{
return value;
}
else
{
return value.length();
}
}
4. Structure Bindings 结构化绑定
C++17 之前
map<string, string> fav_lang{
{ "John", "Java" },
{ "Alex", "C++" },
{ "Peter", "Python" }
};
auto result = fav_lang.insert({ "Henry", "Golang" });
bool success = result.second;
auto process = result.first;
for (const auto& pair : fav_lang) {
const string& name = pair.first;
const string& lang = pair.second;
cout << name << ":" << lang << endl;
}
C++17 之后
map<string, string> fav_lang{
{ "John", "Java" },
{ "Alex", "C++" },
{ "Peter", "Python" }
};
auto[process, success]
= fav_lang.insert({ "Henry",
"Golang" });
for (const auto & [ name, lang ] : fav_lang) {
cout << name << ":"
<< lang << endl;
}
5. Folding Expressions
C++17 之前
auto CPP11_sum()
{
return 0;
}
template<typename T1, typename... T>
auto C11_sum(T1 s, T... ts)
{
return s + CPP11_sum(ts...);
}
C++17 之后
template<typename... Args>
auto sum(Args... args)
{
return (args + ... + 0);
}
template <typename... Args>
auto sum2(Args... args)
{
return (args + ...);
}
6. Direct list initialization of enums
C++17 之前
enum class byte : unsigned char {};
byte b = byte(0); // OK
byte c = byte(-1); // ERROR，-1 超出了 unsigned char 的范围
byte d = byte(1); // OK
byte e = byte(256); // ERROR，256 超出了 unsigned char 的范围
CPP 17 之后
enum byte : unsigned char {};
byte b {0}; // OK
byte c {-1}; // ERROR -1 超出了 unsigned char 的范围
byte d = byte{1}; // OK
byte e = byte{256}; // ERROR 256 超出了 unsigned char 的范围
7. Inline variables 内联变量
全局变量：
// header.h
inline int variable = 42;
// a.cpp
#include "header.h"
// b.cpp
#include "header.h"
类的 inline 变量
// header.h
class myClass {
public:
inline static int var1;
inline static int var2 = 10;
};
inline int myClass::var1 = 5;
8. constexpr lambda 表达式
constexpr auto lamb = [] (int n) { return n * n; };
static_assert(lamb(3) == 9, "a");
9. 新增三个 attributes
[[fallthrough]]
void f(int n)
{
switch (n)
{
case 1:
case 2:
g();
[[fallthrough]]; // 告诉编译器，没有break是正常的，
case 3:
h();
default:
}
}
[[nodiscard]]
[[nodiscard]] int func();
void f()
{
func(); // 会有告警，不要忽略返回值；
}
[[maybe_unused]]
[[maybe_unused]] void f1(){}
void f2(){}
void f3()
{
int x = 0;
[[maybe_unused]] int y = 1; // y没用到，可以消除没用到的告警
return x + 1;
}
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