C++相关闲碎记录(2)
1、误用shared_ptr
int* p = new int;
shared_ptr sp1(p);
shared_ptr sp2(p); //error
// 通过原始指针两次创建shared_ptr是错误的
shared_ptr sp1(new int);
shared_ptr sp2(sp1); //ok 如果对C++相关闲碎记录(1)中记录的shared_ptr的使用例子修改为如下：父类添加孩子的shared_ptr时调用下面这个函数来实现，同样会出现问题：
class Person {
public:
string name;
shared_ptr mother;
shared_ptr father;
vector> kids; //使用weak_ptr
Person(const string& n, shared_ptr m = nullptr,
shared_ptr f = nullptr) :name(n), mother(m), father(f) {}
~Person() {
cout << "delete " << name << endl;
}
void setParentAndTheirKids(shared_ptr m = nullptr, shared_ptr f = nullptr) {
mother = m;
father = f;
if (m != nullptr) {
m->kids.push_back(shared_ptr(this)); //error
// 为什么这里会报错，因为this所指的对象已经有一个shared_ptr了，再通过这种方式创建shared_ptr就会报错，因为会重新开启一个拥有者团队
}
if (f != nullptr){
f->kids.push_back(shared_ptr(this)); //error
}
}
};
shared_ptr initFamily(const string& name) {
shared_ptr mom(new Person(name + "'s mom"));
shared_ptr dad(new Person(name + "'s dad"));
shared_ptr kid(new Person(name));
kid->setParentAndTheirKids(mom, dad);
return kid;
} 使用enable_shared_from_this
#include
#include
#include
#include
using namespace std;
class Person : public enable_shared_from_this {
public:
string name;
shared_ptr mother;
shared_ptr father;
vector> kids; // weak pointer !!!
Person (const string& n)
: name(n) {
}
void setParentsAndTheirKids (shared_ptr m = nullptr,
shared_ptr f = nullptr) {
mother = m;
father = f;
if (m != nullptr) {
m->kids.push_back(shared_from_this());
}
if (f != nullptr) {
f->kids.push_back(shared_from_this());
}
}
~Person() {
cout << "delete " << name << endl;
}
};
shared_ptr initFamily (const string& name)
{
shared_ptr mom(new Person(name+"'s mom"));
shared_ptr dad(new Person(name+"'s dad"));
shared_ptr kid(new Person(name));
kid->setParentsAndTheirKids(mom,dad);
return kid;
}
int main()
{
shared_ptr p = initFamily("nico");
cout << "nico's family exists" << endl;
cout << "- nico is shared " << p.use_count() << " times" << endl;
cout << "- name of 1st kid of nico's mom: "
<< p->mother->kids[0].lock()->name << endl;
p = initFamily("jim");
cout << "jim's family exists" << endl;
} shared_ptr各种操作：
shared_ptr sp(new int);
shared_ptr(static_cast(sp.get())) //error
static_pointer_cast(sp)
std::unique_ptr up = new int; //error
std::unique_ptr up(new int); // ok unique_ptr不必一定拥有对象，也可以是empty。
std::unique_ptr up;
可以赋值为nullptr或者调用reset
up = nullptr;
up.reset(); unique_ptr可以调用release()，释放所拥有的对象，并将所有权交给调用者。
std::unique_ptr up(new std::string("nico"));
std::string* sp = up.release(); 2、转移unique_ptr的拥有权
std::string* sp = new std::string("hello");
std::unique_ptr up1(sp);
std::unique_ptr up(new std::string[10]); //ok
此偏特化不再提供操作符*和->，而提供[]操作符，访问array中的一个对象时，使用[]
std::cout << *up << std::endl; //error
std::cout << up[0] << std::endl; //ok
指定自定义删除器：通过类的方式指定
class ClassADeleter {
public:
void operator() (ClassA* p) {
std::cout << "call delete for ClassA object" << std::endl;
delete p;
}
};
std::unique_ptr up(new ClassA()); 如果是个函数或者lambda，必须声明deleter的类型为void(*)(T*)或者std::function
std::unique_ptr up(new int[10],
[](int* p) {
delete []p;});
std::unique_ptr> up(new int[10],
[](int* p) {
delete []p;});
或者
auto l = [](int*) {delete [] p;};
std::unique_ptr> up(new int[10], l); 为了避免传递function pointer 或者lambda 时必须指明deleter的类型，你可以使用alias template
template
using uniquePtr = std::unique_ptr;
uniquePtr up(new int[10], [](int* p) {delete [] p;}); unique_ptr各种操作：
3、numeric_limits<>
#include
#include
#include
using namespace std;
int main()
{
// use textual representation for bool
cout << boolalpha;
// print maximum of integral types
cout << "max(short): " << numeric_limits::max() << endl;
cout << "max(int): " << numeric_limits::max() << endl;
cout << "max(long): " << numeric_limits::max() << endl;
cout << endl;
// print maximum of floating-point types
cout << "max(float): "
<< numeric_limits::max() << endl;
cout << "max(double): "
<< numeric_limits::max() << endl;
cout << "max(long double): "
<< numeric_limits::max() << endl;
cout << endl;
// print whether char is signed
cout << "is_signed(char): "
<< numeric_limits::is_signed << endl;
cout << endl;
// print whether numeric limits for type string exist
cout << "is_specialized(string): "
<< numeric_limits::is_specialized << endl;
} 4、type trait的使用
#include
#include
#include
using namespace std;
// type trait
template
void foo_impl(T val, true_type) {
std::cout << "Integer" << std::endl;
}
template
void foo_impl(T val, false_type) {
std::cout << "not Integer" << std::endl;
}
template
void foo(T val) {
foo_impl(val, std::is_integral());
}
int main()
{
double d_a = 1.2;
long long int ll_b = 33333;
foo(d_a);
foo(ll_b);
}
输出：
not Integer
Integer 类型判断工具
用以阐明class细节的trait
#include
#include
#include
using namespace std;
int main()
{
std::cout << boolalpha << is_const::value << endl; //false
std::cout << boolalpha << is_const::value << endl; //true
std::cout << boolalpha << is_const::value << endl; //true
std::cout << boolalpha << is_const::value << endl; //false
std::cout << boolalpha << is_const::value << endl; //false
std::cout << boolalpha << is_const::value << endl; //false
std::cout << boolalpha << is_const::value << endl; //true
std::cout << boolalpha << is_const::value << endl; //false
std::cout << boolalpha << is_const::value << endl; //true
return 0;
} 指向const类型的非常量指针或者引用，并不是一个常量，尽管内含元素是常量，例如const int* 并不是常量，只是描述指针所指向的这个变量是常量类型，但是指针本身可以重新指向新的变量。
用以检测copy和move语义的那些个trait，只检测是否相应的表达式为可能，例如一个带有copy构造函数的（接受常量实参）但没有move构造函数的类型，仍然是move constructible.
用以检验类型关系的trait
int main()
{
std::cout << boolalpha << is_assignable::value << endl; //false
std::cout << boolalpha << is_assignable::value << endl; //true
std::cout << boolalpha << is_assignable::value << endl; //false
std::cout << boolalpha << is_assignable::value << endl; //true
std::cout << boolalpha << is_assignable::value << endl; //false
std::cout << boolalpha << is_assignable::value << endl; //false
std::cout << boolalpha << is_assignable::value << endl; //false
std::cout << boolalpha << is_assignable::value << endl; //true
std::cout << boolalpha << is_constructible::value << endl; //true
std::cout << boolalpha << is_constructible::value << endl; //true
std::cout << boolalpha << is_constructible::value << endl; //true
std::cout << boolalpha << is_constructible::value << endl; //false
std::cout << boolalpha << is_constructible::value << endl; //false
std::cout << boolalpha << is_constructible::value << endl; //false
std::cout << boolalpha << is_constructible::value << endl; //true
std::cout << boolalpha << is_constructible::value << endl; //true
return 0;
} 5、类型修饰符
#include
#include
#include
#include
#include
using namespace std;
int main()
{
typedef int T;
typedef add_const::type A; //const int
typedef add_lvalue_reference::type B; //int&
typedef add_rvalue_reference::type C; //int&&
typedef add_pointer::type D; //int*
typedef make_signed::type E; //int
typedef make_unsigned::type F; //unsigned int
typedef remove_const::type G; //int
typedef remove_reference::type H; //int
typedef remove_pointer::type I; //int
std::cout << boolalpha << is_const::value << std::endl;
// 查看完整类型
std::cout << abi::__cxa_demangle(typeid(A).name(),0,0,0 ) << std::endl;
std::cout << typeid(B).name() << std::endl;
std::cout << "A is same const int ?" << boolalpha << is_same::value << std::endl;
std::cout << "B is same int& ?" << boolalpha << is_same::value << std::endl;
typedef const int& T1;
typedef add_const::type A1; // const int&
typedef add_lvalue_reference::type B1; //const int&
typedef add_rvalue_reference::type C1; //const int& (yes, lvalue remains lvalue)
typedef add_pointer::type D1; //const int*
// typedef make_signed::type E1; //undefined behavior
// typedef make_unsigned::type F1; //undefined bahavior
typedef remove_const::type G1; //const int&
typedef remove_reference::type H1; //const int
typedef remove_pointer::type I1; //cosnt int&
std::cout << "A1 is same const int& ?" << boolalpha << is_same::value << std::endl;
std::cout << is_const::value << std::endl;
std::cout << "G1 is same const int& ?" << boolalpha << is_same::value << std::endl;
return 0;
} 
指向某常量类型的reference本身并不是常量，所以不可以移除const，add_pointer<>必然包含使用remove_reference<>，然而make_signed<>和make_unsigned<>必须是整型，枚举型，bool除外，所以传入引用会导致不明确的行为。add_lvalue_reference<>把一个rvalue reference转换为一个lvalue reference，然而add_rvalue_reference<>并不会把一个lvalue reference转换为一个rvalue reference.
6、其他type trait
#include
#include
#include
#include
#include
using namespace std;
int main()
{
std::cout << rank::value << std::endl; //0
std::cout << rank::value << std::endl; //1
std::cout << rank::value << std::endl; //1
std::cout << rank::value << std::endl; //2
std::cout << rank::value << std::endl; //2
std::cout << extent::value << std::endl; //0
std::cout << extent::value << std::endl; //0
std::cout << extent::value << std::endl; //3
std::cout << extent::value << std::endl; //0
std::cout << extent::value << std::endl; //3
std::cout << extent::value << std::endl; //3
std::cout << extent::value << std::endl; //6
std::cout << extent::value << std::endl; //0
typedef remove_extent::type A; //int
typedef remove_extent::type B; //int
typedef remove_extent::type C; //int
typedef remove_extent::type D; //int[8]
typedef remove_extent::type E; //int[7]
typedef remove_all_extents::type F; //int
typedef remove_all_extents::type G; //int
typedef remove_all_extents::type H; //int
typedef remove_all_extents::type I; //int
typedef remove_all_extents::type J; //int
typedef remove_all_extents::type K; //int
return 0;
}