如何从 C 样式数组初始化 std::vector？
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How to initialize std::vector from C-style array?
提问by Frank
What is the cheapest way to initialize a std::vectorfrom a C-style array?
std::vector从 C 样式数组初始化 a 的最便宜的方法是什么？
Example: In the following class, I have a vector, but due to outside restrictions, the data will be passed in as C-style array:
示例：在下面的类中，我有一个vector，但由于外部限制，数据将作为 C 样式数组传入：
class Foo {
std::vector<double> w_;
public:
void set_data(double* w, int len){
// how to cheaply initialize the std::vector?
}
Obviously, I can call w_.resize()and then loop over the elements, or call std::copy(). Are there any better methods?
显然，我可以调用w_.resize()然后循环遍历元素，或者调用std::copy(). 有没有更好的方法？
回答by Pavel Minaev
Don't forget that you can treat pointers as iterators:
不要忘记您可以将指针视为迭代器：
w_.assign(w, w + len);
回答by Mark B
You use the word initialize so it's unclear if this is one-time assignment or can happen multiple times.
您使用初始化一词，因此不清楚这是一次性分配还是可以多次发生。
If you just need a one time initialization, you can put it in the constructor and use the two iterator vector constructor:
如果你只需要一次初始化，你可以把它放在构造函数中并使用两个迭代器向量构造函数：
Foo::Foo(double* w, int len) : w_(w, w + len) { }
Otherwise use assign as previously suggested:
否则按照之前的建议使用assign：
void set_data(double* w, int len)
{
w_.assign(w, w + len);
}
回答by Aaron McDaid
You can 'learn' the size of the array automatically:
您可以自动“学习”数组的大小：
template<typename T, size_t N>
void set_data(const T (&w)[N]){
w_.assign(w, w+N);
}
Hopefully, you can change the interface to set_data as above. It still accepts a C-style array as its first argument. It just happens to take it by reference.
希望您可以将接口更改为 set_data 如上所述。它仍然接受一个 C 风格的数组作为它的第一个参数。它只是碰巧通过引用来获取它。
How it works
这个怎么运作
[ Update: See herefor a more comprehensive discussion on learning the size ]
[更新：有关学习大小的更全面讨论，请参见此处]
Here is a more general solution:
这是一个更通用的解决方案：
template<typename T, size_t N>
void copy_from_array(vector<T> &target_vector, const T (&source_array)[N]) {
target_vector.assign(source_array, source_array+N);
}
This works because the array is being passed as a reference-to-an-array. In C/C++, you cannot pass an array as a function, instead it will decay to a pointer and you lose the size. But in C++, you canpass a reference to the array.
这是有效的，因为数组是作为对数组的引用传递的。在 C/C++ 中，您不能将数组作为函数传递，相反，它会衰减为指针并丢失大小。但是在 C++ 中，您可以传递对数组的引用。
Passing an array by reference requires the types to match up exactly. The size of an array is part of its type. This means we can use the template parameter N to learn the size for us.
通过引用传递数组需要类型完全匹配。数组的大小是其类型的一部分。这意味着我们可以使用模板参数 N 来为我们学习大小。
It might be even simpler to have this function which returns a vector. With appropriate compiler optimizations in effect, this should be faster than it looks.
使用这个返回向量的函数可能更简单。通过适当的编译器优化，这应该比看起来更快。
template<typename T, size_t N>
vector<T> convert_array_to_vector(const T (&source_array)[N]) {
return vector<T>(source_array, source_array+N);
}
回答by Mugurel
Well, Pavel was close, but there's even a more simple and elegant solution to initialize a sequential container from a c style array.
嗯，Pavel 很接近，但是还有一个更简单和优雅的解决方案来从 ac 样式数组初始化顺序容器。
In your case:
在你的情况下：
w_ (array, std::end(array))
- array will get us a pointer to the beginning of the array (didn't catch it's name),
- std::end(array) will get us an iterator to the end of the array.
- array 会给我们一个指向数组开头的指针（没有抓住它的名字），
- std::end(array) 会给我们一个迭代器到数组的末尾。
回答by eusoubrasileiro
回答by Janusz Lenar
std::vector<double>::assignis the way to go, because it's little code. But how does it work, actually? Doesnt't it resize and then copy? In MS implementation of STL I am using it does exactly so.
std::vector<double>::assign是要走的路，因为它的代码很少。但它实际上是如何工作的？它不是调整大小然后复制吗？在 STL 的 MS 实现中，我正在使用它正是如此。
I'm afraid there's no faster wayto implement (re)initializing your std::vector.
恐怕没有更快的方法来实现（重新）初始化您的std::vector.
