diff --git a/README.md b/README.md
index 2c0323e..3b1a993 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 ### 0.项目概要
 
-学习C++内容，包括理论、源码、实践、课程代码、项目等
+学习C++内容，包括理论、源码、实践、课程代码、项目等。
 
 ### 1.基础部分
 
@@ -146,6 +146,7 @@ for(decl:col) {
 - [是不是应该返回对象？](./modern_C++_30/returnObj)
 - [编译期多态：泛型编程和模板入门](./modern_C++_30/compilerpoly)
 - [SFINAE：不是错误的替换失败是怎么回事?](./modern_C++_30/SFINAE)
+- [constexpr：一个常态的世界](./modern_C++_30/constexpr)
 
 ### 4.拓展部分
 
diff --git a/modern_C++_30/.CMakeLists.txt.un~ b/modern_C++_30/.CMakeLists.txt.un~
index 65349bb..49bafb8 100644
Binary files a/modern_C++_30/.CMakeLists.txt.un~ and b/modern_C++_30/.CMakeLists.txt.un~ differ
diff --git a/modern_C++_30/CMakeLists.txt b/modern_C++_30/CMakeLists.txt
index 96a917d..b29314f 100644
--- a/modern_C++_30/CMakeLists.txt
+++ b/modern_C++_30/CMakeLists.txt
@@ -62,3 +62,7 @@ add_executable(lambda "SFINAE/sfinae paper/lambda.cpp")
 add_executable(is_valid "SFINAE/sfinae paper/is_valid.cpp")
 add_executable(fis_valid "SFINAE/sfinae paper/fis_valid.cpp")
 add_executable(packis_valid "SFINAE/sfinae paper/packis_valid.cpp")
+
+
+add_executable(test3 constexpr/test3.cpp)
+add_executable(sqrt constexpr/sqrt.cpp)
diff --git a/modern_C++_30/CMakeLists.txt~ b/modern_C++_30/CMakeLists.txt~
new file mode 100644
index 0000000..96a917d
--- /dev/null
+++ b/modern_C++_30/CMakeLists.txt~
@@ -0,0 +1,64 @@
+cmake_minimum_required(VERSION 3.14)
+project(Morden_C++)
+
+set(CMAKE_CXX_STANDARD 17)
+
+add_executable(heap RAII/heap.cpp)
+add_executable(stack RAII/stack.cpp)
+add_executable(RAII RAII/RAII.cpp)
+add_executable(RAII_fstram RAII/RAII_fstram.cpp)
+
+add_executable(auto_scope smart_ptr/auto_scope.cpp)
+add_executable(unique_ptr smart_ptr/unique_ptr.cpp)
+add_executable(unique_ptr_U smart_ptr/unique_ptr_U.cpp)
+add_executable(shared_ptr smart_ptr/shared_ptr.cpp)
+
+add_executable(reference reference/reference.cpp)
+add_executable(forward reference/forward.cpp)
+add_executable(collapses reference/collapses.cpp)
+add_executable(lifetime reference/lifetime.cpp)
+add_executable(dontreturnReference reference/don'treturnReference.cpp)
+
+add_executable(container container1/container.cpp)
+add_executable(cont container2/hash.cpp)
+add_executable(vector_l container1/vector_l.cpp)
+add_executable(priority_queue container2/priority_queue.cpp)
+add_executable(relacontainer container2/relacontainer.cpp)
+add_executable(unorder container2/unorder.cpp)
+add_executable(array container2/array.cpp)
+add_executable(exception exception/exception.cpp)
+
+
+add_executable(returnObj returnObj/returnObj1.cpp)
+add_executable(returnObj_add returnObj/returnObj2.cpp)
+add_executable(returnObj3 returnObj/returnObj3.cpp)
+add_executable(returnObj4 returnObj/returnObj4.cpp)
+add_executable(returnObj5 returnObj/returnObj5.cpp)
+add_executable(rvonrvo returnObj/all.cpp)
+
+add_executable(literal literalAssert/literal.cpp)
+add_executable(assert literalAssert/assert.cpp)
+add_executable(default_delete literalAssert/default_delete.cpp)
+add_executable(overridefinal literalAssert/overridefinal.cpp)
+
+
+add_executable(SFINAE "SFINAE/SFINAE.cpp")
+add_executable(serialize "SFINAE/sfinae paper/serialize.cpp")
+add_executable(hana "SFINAE/sfinae paper/hana.cpp")
+add_executable(overload "SFINAE/sfinae paper/overload1.cpp")
+add_executable(pSFINAE "SFINAE/sfinae paper/p1SFINAE.cpp")
+add_executable(p1SFINAE "SFINAE/sfinae paper/p2SFINAE.cpp")
+add_executable(sizeof1 "SFINAE/sfinae paper/sizeof1.cpp")
+add_executable(sizeof2 "SFINAE/sfinae paper/sizeof2.cpp")
+add_executable(combining "SFINAE/sfinae paper/combiningAndGenius.cpp")
+add_executable(overload2 "SFINAE/sfinae paper/overload2.cpp")
+add_executable(combining1 "SFINAE/sfinae paper/timeGenius.cpp")
+add_executable(decltype "SFINAE/sfinae paper/decltype.cpp")
+add_executable(auto "SFINAE/sfinae paper/auto.cpp")
+add_executable(constexpr "SFINAE/sfinae paper/constexpr.cpp")
+add_executable(blending "SFINAE/sfinae paper/blending1.cpp")
+add_executable(blending2 "SFINAE/sfinae paper/blending2.cpp")
+add_executable(lambda "SFINAE/sfinae paper/lambda.cpp")
+add_executable(is_valid "SFINAE/sfinae paper/is_valid.cpp")
+add_executable(fis_valid "SFINAE/sfinae paper/fis_valid.cpp")
+add_executable(packis_valid "SFINAE/sfinae paper/packis_valid.cpp")
diff --git a/modern_C++_30/constexpr/a.out b/modern_C++_30/constexpr/a.out
new file mode 100755
index 0000000..011df3e
Binary files /dev/null and b/modern_C++_30/constexpr/a.out differ
diff --git a/modern_C++_30/constexpr/container.cpp b/modern_C++_30/constexpr/container.cpp
new file mode 100644
index 0000000..bf2cd74
--- /dev/null
+++ b/modern_C++_30/constexpr/container.cpp
@@ -0,0 +1,43 @@
+//
+// Created by light on 19-12-16.
+//
+
+#include <iostream>
+#include <map>
+#include <set>
+#include <vector>
+#include "output_container.h"
+
+
+using namespace std;
+
+int main() {
+    map<int, int> mp{
+            {1, 1},
+            {2, 4},
+            {3, 9}};
+    cout << mp << endl;
+    vector<vector<int>> vv{
+            {1, 1},
+            {2, 4},
+            {3, 9}};
+    cout << vv << endl;
+
+    pair<int, int> p{1, 2};
+    cout << p << endl;
+
+    set<int> s{1, 2, 3};
+    cout << s << endl;
+
+    vector<char> v{'a', 'b'};
+    cout << v << endl;
+    set<char *> vs{"a", "b"};
+    cout << vs << endl;
+
+    map<int, char *> mm{
+            {1,   "23"},
+            {2, "234hi"}
+    };
+    cout << mm << endl;
+
+}
diff --git a/modern_C++_30/constexpr/newconstexpr.cpp b/modern_C++_30/constexpr/newconstexpr.cpp
new file mode 100644
index 0000000..b8b9570
--- /dev/null
+++ b/modern_C++_30/constexpr/newconstexpr.cpp
@@ -0,0 +1,78 @@
+//
+// Created by light on 20-1-7.
+//
+
+#include <iostream>
+#include <vector>
+#include <array>
+
+using namespace std;
+
+// C++17 内联变量
+struct magic {
+//    static const int number = 42;  // error
+    static constexpr int number = 42; // ok
+// 类的静态constexpr成员变量默认就是内联的。
+//    static inline const int number = 42; // ok
+//  const常量和类外面的constexpr变量是不默认内联，需要手工加inline关键字才会变成内联
+};
+
+// 内联变量之前用这个方法,称为one definition rule，有了内联变量及模板，则不受这条规则限制!
+// const int magic::number = 42;  // ok
+
+
+// C++14 变量模板
+// 之前我们需要用类静态数据成员来表达的东西，使用变量模板可以更简洁地表达。
+namespace my {
+    template<class T>
+    inline constexpr bool
+            is_trivially_destructible_v =
+            std::is_trivially_destructible<
+                    T>::value;
+}
+// 不重要析构
+class A {
+};
+
+// 重要析构
+class B {
+    ~B() {}
+};
+
+int main() {
+    std::cout << magic::number << std::endl;
+
+    std::vector<int> v;
+    v.push_back(magic::number); // undefined reference to `magic::number'
+    std::cout << v[0] << std::endl;
+    // 不重要的析构
+    std::cout << is_trivially_destructible_v<A> << std::endl;
+    std::cout << is_trivially_destructible_v<B> << std::endl;
+
+    // constexpr变量仍是const
+
+    constexpr int a = 42;
+    const int &b = a;
+    // 上述const不能去掉,否则报如下错误：
+    // binding reference of type ‘int&’ to ‘const int’ discards qualifiers
+
+
+    // constexpr 构造函数和字面类型
+    // C++14 放开了 constexpr函数里的循环
+    // C++20 放开了 try...catch语句 和 asm声明
+    // constexpr函数里不能使用goto语句
+
+    // 一个有意思的情况是一个类的构造函数。如果一个类的构造函数里面只包含常量表达式、
+    // 满足对 constexpr 函数的限制的话（这也意味着，里面不可以有任何动态内存分配），
+    // 并且类的析构函数是平凡的，那这个类就可以被称为是一个字面类型。
+    std::array<int, 5> ay;
+
+
+    constexpr string_view sv{"hi"};
+    constexpr pair pr{sv[0], sv[1]};
+    constexpr array aa{pr.first, pr.second};
+    constexpr int n1 = aa[0];
+    constexpr int n2 = aa[1];
+    cout << n1 << ' ' << n2 << '\n';
+
+}
\ No newline at end of file
diff --git a/modern_C++_30/constexpr/output_container.h b/modern_C++_30/constexpr/output_container.h
new file mode 100644
index 0000000..f7f038f
--- /dev/null
+++ b/modern_C++_30/constexpr/output_container.h
@@ -0,0 +1,150 @@
+//
+// Created by light on 20-1-8.
+//
+
+#ifndef MORDEN_C_OUTPUT_CONTAINER_H
+#define MORDEN_C_OUTPUT_CONTAINER_H
+
+#include <ostream>      // std::ostream
+#include <type_traits>  // std::false_type/true_type/decay_t/is_same_v
+#include <utility>      // std::declval/pair
+
+// 检测是否是pair
+template<typename T>
+struct is_pair : std::false_type {
+};
+template<typename T, typename U>
+struct is_pair<std::pair<T, U>> : std::true_type {
+};
+template<typename T>
+inline constexpr bool is_pair_v = is_pair<T>::value;
+
+// 检测是否可以直接输出
+template<typename T>
+struct has_output_function {
+    template<class U>
+    static auto output(U *ptr)
+    -> decltype(std::declval<std::ostream &>() << *ptr,
+            std::true_type());
+
+    template<class U>
+    static std::false_type output(...);
+
+    static constexpr bool value =
+            decltype(output<T>(nullptr))::value;
+};
+
+template<typename T>
+inline constexpr bool has_output_function_v =
+        has_output_function<T>::value;
+
+
+enum CHARS {
+    ORD,        // 其他类型
+    CHAR,       // char 类型
+    STRING      // string 类型
+};
+
+template<typename T>
+int ischarOrString(T &elem) {
+    using std::decay_t;
+    using std::is_same_v;
+    using element_type = decay_t<decltype(elem)>;
+    constexpr bool is_char_v = is_same_v<element_type, char>;
+    constexpr bool is_string_v = is_same_v<element_type, char *> ||
+                                 is_same_v<element_type, const char *> ||
+                                 is_same_v<element_type, std::string>;
+    if (is_char_v)
+        return CHAR;
+    else if (is_string_v)
+        return STRING;
+    else
+        return ORD;
+}
+
+template<typename T>
+void output(T &elem, int type, std::ostream &os) {
+    switch (type) {
+        case CHAR:
+            os << '\'' << elem << '\'';
+            break;
+        case STRING:
+            os << '\"' << elem << '\"';
+            break;
+        case ORD:
+            os << elem;
+            break;
+    }
+}
+
+template<typename T, typename Cont>
+auto output_element(std::ostream &os, const T &element,
+                    const Cont &)
+-> typename std::enable_if<is_pair<typename Cont::value_type>::value, bool>::type {
+    int ftype = ischarOrString(element.first);
+    int stype = ischarOrString(element.second);
+
+    output(element.first, ftype, os);
+    os << " => ";
+    output(element.second, stype, os);
+    return true;
+}
+
+template<typename T, typename Cont>
+auto output_element(std::ostream &os, const T &element,
+                    const Cont &)
+-> typename std::enable_if<!is_pair<typename Cont::value_type>::value, bool>::type {
+    int etype = ischarOrString(element);
+    output(element, etype, os);
+    return false;
+}
+
+template<typename T, typename U>
+std::ostream &operator<<(std::ostream &os, const std::pair<T, U> &pr) {
+    os << '(' << pr.first << ", " << pr.second << ')';
+    return os;
+}
+
+//template<typename T, typename Cont>
+//auto output_element(std::ostream& os, const T& element,
+//                    const Cont&, const std::true_type)
+//-> decltype(std::declval<typename Cont::key_type>(), os)
+//{
+//    os << element.first << " => " << element.second;
+//    return os;
+//}
+//
+//template <typename T, typename Cont>
+//auto output_element(std::ostream& os, const T& element,
+//                    const Cont&, ...)
+//-> decltype(os)
+//{
+//    os << element;
+//    return os;
+//}
+
+
+
+// 针对没有输出函数的容器处理
+template<typename T,
+        typename = std::enable_if_t<!has_output_function_v<T>>>
+auto operator<<(std::ostream &os, const T &container)
+-> decltype(container.begin(), container.end(), os) {
+    os << "{ ";
+    if (!container.empty()) {
+        bool on_first_element = true;
+        for (auto elem:container) {
+            if (!on_first_element) {
+                os << ", ";
+            } else {
+                on_first_element = false;
+            }
+            output_element(os, elem, container);
+        }
+    }
+    os << " }";
+    return os;
+}
+
+
+#endif //MORDEN_C_OUTPUT_CONTAINER_H
diff --git a/modern_C++_30/constexpr/sqrt.cpp b/modern_C++_30/constexpr/sqrt.cpp
new file mode 100644
index 0000000..6a12987
--- /dev/null
+++ b/modern_C++_30/constexpr/sqrt.cpp
@@ -0,0 +1,47 @@
+//
+// Created by light on 20-1-7.
+//
+
+#include <iostream>
+#include <array>
+
+int sqr(int n) {
+    return n * n;
+}
+
+int rand() {
+    return std::rand();
+}
+
+// constant expression
+constexpr int sqr1(int n) {
+    return n * n;
+}
+
+constexpr int factorial(int n) {
+    // n是普通的int 不能使用static_assert
+//    static_assert(n>=0); //error
+    // 正确方式
+    if (n < 0) {
+        throw std::invalid_argument(
+                "Arg must be non-negative");
+    }
+
+    if (n == 0) {
+        return 1;
+    } else {
+        return n * factorial(n - 1);
+    }
+}
+
+int main() {
+    int a[sqr(3)];  // ok
+    const int n1 = sqr(3);  // ok
+
+    constexpr int n = sqr1(3); // constexpr=constant expression 常量表达式
+    std::array<int, n> aa;  // ok
+
+    std::array<int, factorial(10)> b;   // ok
+    int cc[rand()];
+    return 0;
+}
\ No newline at end of file
diff --git a/modern_C++_30/constexpr/test3.cpp b/modern_C++_30/constexpr/test3.cpp
new file mode 100644
index 0000000..73e8924
--- /dev/null
+++ b/modern_C++_30/constexpr/test3.cpp
@@ -0,0 +1,31 @@
+//
+// Created by light on 20-1-10.
+//
+
+#include <iostream>
+
+class test3 {
+public:
+
+    int value;
+
+    // constexpr const method - can't chanage the values of object fields and can be evaluated at compile time.
+    constexpr int getvalue() const {
+        return (value);
+    }
+
+    constexpr test3(int Value)
+            : value(Value) {
+    }
+};
+
+
+int main() {
+
+    // 加不加都行
+    constexpr test3 x(100); // OK. Constructor is constexpr.
+
+    int array[x.getvalue()]; // OK. x.getvalue() is constexpr and can be evaluated at compile time.
+}
+
+