Effective Modern C++

C++ 11.jpeg

1, left value vs right value

In concept (though not always in practice), rvalues correspond to temporary objects returned from functions, while lvalues correspond to objects you can refer to, either by name or by following a pointer or lvalue reference. A useful heuristic to determine whether an expression is an lvalue is to ask if you can take its address.

2, auto

推断规则同template，如下

template<T>
void fun(paratype )

fun(expr)
auto a = 1;
const auto& b = a;   //auto = T

特例

auto a= {1,2,3}   //支持initilizer_list
fun({1,2,3})        //不支持，没啥原因，just sb rules

auto 在一些不可见的代理类型中会出错，比如vector<bool>的底层实现是bit

3, safe design

• nullptr vs 0, NULL

void fun(int)
void fun(void*)

fun(0)            //fun(int)
fun(NULL)    //fun(int)
fun(nullptr)   //fun(void*)
C++ 中 #define NULL 0， 容易产生二义性

4, move semantics perfect forwarding

move: unconditionally cast to right value reference (static<T&&> para)
perfect forwarding: cast to right value reference only the parameter is initialized with right value ref

void process(const Widget& lvalArg); // process lvalues
void process(Widget&& rvalArg); // process rvalues

template<typename T> // template that passes
void logAndProcess(T&& param) // param to process
{
  auto now = // get current time
  std::chrono::system_clock::now();
  makeLogEntry("Calling 'process'", now);
  process(std::forward<T>(param));
}

Widget w;
logAndProcess(w); // call with lvalue
logAndProcess(std::move(w)); // call with rvalue