有没有更有效的方法来从流中设置std :: vector? [英] Is there a more efficient way to set a std::vector from a stream?

问题描述

目前，我从 std :: ostringstream 设置 std :: vector< char> 的值，如下所示:

Presently, I set the value of a std::vector<char> from an std::ostringstream as follows:

void
foo(std::vector<char> &data, std::stringstream &stream) {
  data = std::vector<char>(stream.str().begin(), stream.str().end());
}

我想知道是否有更有效的方法可以使用C ++中的STL来执行此操作，或者我在此给出的方法是否合适?我会改用 std :: stringstream 更好吗?

I'm wondering if there is a more efficient way to do this with STL in C++ or whether the method I give here is considered appropriate? Would I be better off using std::stringstream instead?

推荐答案

您的方法调用未定义的行为. stream.str()返回一个字符串 by-value ，也就是一个临时字符串.您使用一个临时对象的 begin 迭代器，以及另一个临时对象的 end 迭代器，创建一个无效范围.

Your method invokes undefined behaviour. stream.str() returns a string by-value, aka a temporary string. You take the begin iterator of one temporary and the end iterator of the other, creating an invalid range.

将流转换为容器的一种方法是使用通用的迭代器接口:

One method to convert a stream to a container is to use the common iterator interface:

#include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <iterator>

int main(){
  std::stringstream src("....");
  std::vector<char> dest;
  // for a bit of efficiency
  std::streampos beg = src.tellg();
  src.seekg(0, std::ios_base::end);
  std::streampos end = src.tellg();
  src.seekg(0, std::ios_base::beg);
  dest.reserve(end - beg);

  dest.assign(std::istreambuf_iterator<char>(src), std::istreambuf_iterator<char>());

  std::copy(dest.begin(), dest.end(), std::ostream_iterator<char>(std::cout));
}

Ideone上的实时示例.

另一种方法是缓存返回的 std :: string 对象:

Another method would be to cache the returned std::string object:

std::string const& s = stream.str();
data.reserve(s.size());
data.assign(s.begin(), s.end());

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