只读操作的 std::map 线程安全

问题描述

我有一个 std::map 用于将值(字段 ID)映射到人类可读的字符串.当我的程序在启动任何其他线程之前启动时，此映射会被初始化一次，之后就再也不会被修改了.现在，我为每个线程提供了这个(相当大的)映射的自己的副本，但这显然是内存使用效率低下并且会减慢程序启动速度.所以我想给每个线程一个指向映射的指针，但这引发了一个线程安全问题.

I have a std::map that I use to map values (field ID's) to a human readable string. This map is initialised once when my program starts before any other threads are started, and after that it is never modified again. Right now, I give every thread its own copy of this (rather large) map but this is obviously inefficient use of memory and it slows program startup. So I was thinking of giving each thread a pointer to the map, but that raises a thread-safety issue.

如果我所做的只是使用以下代码读取地图:

If all I'm doing is reading from the map using the following code:

std::string name;
//here N is the field id for which I want the human readable name
unsigned field_id = N;
std::map<unsigned,std::string>::const_iterator map_it;

// fields_p is a const std::map<unsigned, std::string>* to the map concerned.
// multiple threads will share this.
map_it = fields_p->find(field_id);
if (map_it != fields_p->end())
{
    name = map_it->second;
}
else
{
    name = "";
}

这会起作用还是从多个线程读取 std::map 有问题?

Will this work or are there issues with reading a std::map from multiple threads?

注意:我目前正在使用 Visual Studio 2008，但我希望它适用于大多数主要的 STL 实现.

Note: I'm working with visual studio 2008 currently, but I'd like this to work acros most main STL implementations.

更新:编辑代码示例以确保常量正确性.

Update: Edited code sample for const correctness.

推荐答案

只要您的地图保持不变，这将适用于多个线程.您使用的地图实际上是不可变的，因此任何查找实际上都会在不会更改的地图中进行查找.

This will work from multiple threads as long as your map remains the same. The map you use is immutable de facto so any find will actually do a find in a map which does not change.

这是一个相关链接:http://www.sgi.com/tech/stl/thread_safety.html

STL 的 SGI 实现是线程安全仅在这种意义上同时访问不同的容器是安全的，同时对共享容器的读访问是安全的.如果多个线程访问一个单个容器，以及至少一个线程可能会写，那么用户有责任确保线程之间的互斥在容器访问期间.

您属于对共享容器的同时读取访问"类别.

You fall into he "simultaneous read accesses to shared containers" category.

注意:这适用于 SGI 实现.您需要检查是否使用其他实现.在这两种似乎被广泛用作替代方案的实现中，据我所知，STLPort 具有内置的线程安全性.不过我不知道 Apache 的实现.

Note: this is true for the SGI implementation. You need to check if you use another implementation. Of the two implementations which seem widely used as an alternative, STLPort has built-in thread safety as I know. I don't know about the Apache implementation though.

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