如何在 C++ 中初始化向量 [英] How to initialize a vector in C++

问题描述

我想初始化一个向量，就像我们在数组的情况下所做的那样.

I want to initialize a vector like we do in case of an array.

示例

int vv[2] = {12, 43};

但是当我这样做时，

vector<int> v(2) = {34, 23};

或

vector<int> v(2);
v = {0, 9};

它给出了一个错误:

'{' 标记前的预期主表达式

expected primary-expression before ‘{’ token

和

错误:在="标记之前应为,"或;"

error: expected ‘,’ or ‘;’ before ‘=’ token

分别.

推荐答案

使用新的 C++ 标准(可能需要在编译器上启用特殊标志)，您可以简单地执行以下操作:

With the new C++ standard (may need special flags to be enabled on your compiler) you can simply do:

std::vector<int> v { 34,23 };
// or
// std::vector<int> v = { 34,23 };

甚至:

std::vector<int> v(2);
v = { 34,23 };

在不支持这个特性(初始化列表)的编译器上，你可以用一个数组来模拟它:

On compilers that don't support this feature (initializer lists) yet you can emulate this with an array:

int vv[2] = { 12,43 };
std::vector<int> v(&vv[0], &vv[0]+2);

或者，对于分配给现有向量的情况:

Or, for the case of assignment to an existing vector:

int vv[2] = { 12,43 };
v.assign(&vv[0], &vv[0]+2);

就像 James Kanze 建议的那样，拥有提供数组开头和结尾的函数会更健壮:

Like James Kanze suggested, it's more robust to have functions that give you the beginning and end of an array:

template <typename T, size_t N>
T* begin(T(&arr)[N]) { return &arr[0]; }
template <typename T, size_t N>
T* end(T(&arr)[N]) { return &arr[0]+N; }

然后你就可以做到这一点，而不必全部重复大小:

And then you can do this without having to repeat the size all over:

int vv[] = { 12,43 };
std::vector<int> v(begin(vv), end(vv));

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