调整C ++ std :: vector< char>而不初始化数据 [英] Resizing a C++ std::vector<char> without initializing data

问题描述

对于向量，可以假设元素在存储器中连续存储，允许范围 [& vec [0]，& vec [vec.capacity（））正常数组。例如

With vectors, one can assume that elements are stored contiguously in memory, allowing the range [&vec[0], &vec[vec.capacity()) to be used as a normal array. E.g.,

vector<char> buf;
buf.reserve(N);
int M = read(fd, &buf[0], N);

但是现在向量不知道它包含 M 字节数据，由 read（）外部添加。我知道 vector :: resize（）设置大小，但它也清除数据，所以它不能用于更新大小之后 read（）调用。

But now the vector doesn't know that it contains M bytes of data, added externally by read(). I know that vector::resize() sets the size, but it also clears the data, so it can't be used to update the size after the read() call.

有没有一种简单的方法可以直接读取数据到向量中并更新大小？是的，我知道明显的解决方法，如使用一个小数组作为临时读缓冲区，并使用 vector :: insert（）将它附加到向量的末尾：

Is there a trivial way to read data directly into vectors and update the size after? Yes, I know of the obvious workarounds like using a small array as a temporary read buffer, and using vector::insert() to append that to the end of the vector:

char tmp[N];
int M = read(fd, tmp, N);
buf.insert(buf.end(), tmp, tmp + M)

工作（这是我今天做的），但它只是困扰我，有一个额外的复制操作，如果我可以直接把数据到向量中，不需要。

This works (and it's what I'm doing today), but it just bothers me that there is an extra copy operation there that would not be required if I could put the data directly into the vector.

那么，当外部添加数据时，是否有一个简单的方法来修改向量大小？

So, is there a simple way to modify the vector size when data has been added externally?

推荐答案

vector<char> buf;
buf.reserve(N);
int M = read(fd, &buf[0], N);

此代码段调用未定义的行为。即使您已保留空格，也不能写入 size（）元素。

This code fragment invokes undefined behavior. You can't write beyond than size() elements, even if you have reserved the space.

正确的代码如下：

vector<char> buf;
buf.resize(N);
int M = read(fd, &buf[0], N);
buf.resize(M);

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向量，可以假设元素被连续地存储在存储器中，允许范围 [& vec [0]，& vec [vec.capacity（）） 用作正常数组不是真的。允许范围为 [& vec [0]，& vec [vec.size（）） 。

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PS. Your statement "With vectors, one can assume that elements are stored contiguously in memory, allowing the range [&vec[0], &vec[vec.capacity()) to be used as a normal array" isn't true. The allowable range is [&vec[0], &vec[vec.size()).

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