初始化initializer list
Prerequisites:
維基介紹 初始化串列
整數
c++11用大括號{},等於號可省略
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
| #include <iostream>
using namespace std;
int main() {
int a = 10;
// c++98,用圓括號()
int b = (10);
int c(10);
cout << "a = " << a << endl;
cout << "b = " << b << endl;
cout << "c = " << c << endl;
//c++11用大括號{},等於號可省略
int d = {10};
int e{10};
cout << "d = " << d << endl;
cout << "e = " << e << endl;
return 0;
}
|
a = 10
b = 10
c = 10
d = 10
e = 10
可以用 =、() 或 {}，以下用法都對
1
2
3
4
5
6
| int x = 3;
int x(3);
int x{3};
string name("Some Name");
string name = "Some Name";
string name{"Some Name"};
|
浮點數
1
2
| double f{1.12};
cout << "f = " << f << endl;
|
此外，{} 初始列不允許整數型別的縮小 (narrowing) 轉換，這可以用來避免一些型別上的程式撰寫錯誤。
1
2
| int pi(3.14); // 可 -- pi == 3.
int pi{3.14}; // 編譯器錯誤：縮小轉換
|
陣列
1
2
3
4
5
| //陣列
int arr1[5]{1, 2, 3, 4, 5};
for (int i = 0; i < 5; i++) {
cout << "arr1[" << i << "] = " << arr1[i] << endl;
}
|
arr1[0] = 1
arr1[1] = 2
arr1[2] = 3
arr1[3] = 4
arr1[4] = 5
new動態分配記憶體位址
1
2
3
4
| int *arr_p = new int[5]{11, 12, 13, 14, 15};
for (int i = 0; i < 5; i++) {
cout << "arr_p[" << i << "] = " << arr_p[i] << endl;
}
|
arr_p[0] = 11
arr_p[1] = 12
arr_p[2] = 13
arr_p[3] = 14
arr_p[4] = 15
建立物件
c++11用大括號{},等於號可省略
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
| #include <iostream>
using namespace std;
class Student {
public:
Student(int number, string name) : number(number), name(name){}
private:
int number; // 學號
string name;
};
int main() {
// c98
Student s1(10122, "Bill");
// c11
Student s2 = {10123, "Mary"};
//c11 省略等於號
Student s3{10124, "Alice"};
return 0;
}
|
初始化容器
1
2
3
4
5
6
7
8
9
10
11
12
| #include <iostream>
#include <vector>
using namespace std;
int main() {
// 把v1初始化為10個元素
vector<int> v1(10);
// v2只有1個元素10
vector<int> v2{10};
// v3有3個元素，分別是11 12 13
vector<int> v3{11, 12, 13};
return 0;
}
|
1
2
| std::vector<int> v(100, 1); // vector 中有 100 個元素：每個元素都是 1
std::vector<int> v{100, 1}; // vector 中有 2 個元素：100 和 1
|
函式參數
可以把大括號{}間的值，作為參數傳給函式。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
| #include <iostream>
using namespace std;
int func1(initializer_list<int> init_list) {
int total = 0;
for (int val : init_list) {
total += val;
cout << val << ",";
}
cout << endl;
return total;
}
int main() {
int total = func1({1, 2, 3, 4, 5});
cout << "total = " << total << endl;
return 0;
}
|
initializer_list模板類別，跟容器一樣，提供begin()與end()
編譯
若在linux編譯時，需要加上-std=c++11
g++ -o c11_init c11_init.cpp -std=c++11