# C++ 11

## C++ 11

### auto

```cpp
auto a; // wrong
auto i = 1; 
auto d = 1.0; 
auto str = "Hello World"; 
auto ch = 'A'; 
auto func = less<int>(); 
vector<int> iv; 
auto ite = iv.begin();
auto p = new foo() // customized type
```

### initialization

```cpp
int arr[3]{1, 2, 3}; 
vector<int> v{1, 2, 3}; 
map<int, string> m{{1, "a"}, {2, "b"}};
string str{"Hello World"};
```

### for loop

```cpp
map<string, int> m{{"a", 1}, {"b", 2}, {"c", 3}}; 
for (auto p : m){ 
    cout<<p.first<<" : "<<p.second<<endl;
}
```

### tuple (variable-length template)

```cpp
auto p = make_pair(1, "before C++ 11");

auto t1 = make_tuple(1, 2.0, "C++ 11"); 
auto t2 = make_tuple(1, 2.0, "C++ 11", {1, 0, 2});
```

### lambda expression

```cpp
vector<int> iv{5, 4, 3, 2, 1}; 
int a = 2, b = 1;
// syntax: [global param](func args)->return_type{func body} 
for_each(iv.begin(), iv.end(), [b](int &x){cout<<(x + b)<<endl;});
for_each(iv.begin(), iv.end(), [=](int &x){x *= (a + b);});
for_each(iv.begin(), iv.end(), [=](int &x)->int{return x * (a + b);});
```

### decltype

```cpp
template <typename Creator> 
auto processProduct(const Creator& creator) -> 
decltype(creator.makeObject()) { 
    auto val = creator.makeObject(); 
    // do something with val
}
```

### nullptr

```cpp
void F(int a){
    cout << a << endl;
} 
void F(int *p){
    assert(p != NULL); 
    cout << p << endl;
} 
int main(){
    int *p = nullptr; 
    int *q = NULL; 
    int a = nullptr;  // compilation error; not allowed
    bool equal = ( p == q );  // true
    F(0);  // compilation error in C++98 due to ambiguity; call F(int) in C++11
    F(nullptr);
    return 0;
}
```

### rvalue reference; move semantics; perfect forwarding

```cpp
#include <iostream> 
#include <utility> 
void reference(int& v) { 
    std::cout << "lvalue" << std::endl;
} 
void reference(int&& v) { 
    std::cout << "rvalue" << std::endl;
} 
template <typename T> 
void pass(T&& v) { 
    std::cout << "regular: "; 
    reference(v); 
    std::cout << "std::move: "; 
    reference(std::move(v)); 
    std::cout << "std::forward: "; 
    reference(std::forward<T>(v)); // cf. static_cast<T&&>(v)
} 
int main() { 
    std::cout << "pass rvalue:" << std::endl; 
    pass(1);
    
    std::cout << "pass lvalue:" << std::endl; 
    int v = 1; 
    pass(v);
    
    return 0;
}
/* Output:
pass rvalue:
regular: lvalue
std::move: rvalue
std::forward: rvalue
pass lvalue:
regular: lvalue
std::move: rvalue
std::forward: lvalue
*/

```


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