Is std::vector or boost::vector Thread Safe?

In C++, a common question that arises is: Is std::vector or boost::vector thread-safe? The straightforward answer is no, neither std::vector nor boost::vector is thread-safe.

In this article, we will learn why std::vector and boost::vector are not thread-safe and explore some practical alternatives for making them safe to use in a multi-threaded environment.

What is Thread Safety?

Thread safety refers to the property of a program or code segment that ensures correct behavior when accessed from multiple threads simultaneously. A thread-safe component can be used by multiple threads concurrently without causing data races or inconsistent states.

Problems with Thread Safety in std::vector and boost::vector

is a part of the widely used for dynamic array implementations whereas boost::vector is part of the , designed to extend the functionalities of the C++ Standard Library. However, both std::vector and boost::vector are not inherently thread-safe.

1. Concurrent Read-Write Access

Simultaneous read and write operations on the same std::vector or boost::vector instance can cause undefined behavior due to data races.

2. Concurrent Write Access

Simultaneous write operations (e.g., modifying, inserting, or deleting elements) are not safe without external synchronization mechanisms.

3. Lack of Built-in Synchronization

Neither std::vector nor boost::vector provides built-in mechanisms to handle concurrent access, which means the user must handle synchronization.

How to Ensure Thread Safety with std::vector and boost::vector

To make std::vector or boost::vector thread-safe, we need to use like mutexes.

For std::vector

We can use a to ensure mutual exclusion, which prevents multiple threads from accessing the vector simultaneously. Using this we can protect read and write operations to ensure mutual exclusion.

Example:

Output

Reading from vector: 
Writing to vector completed.
Reading from vector: 0 1 2 3 4 5 6 7 8 9 

For boost::vector

Using boost::vector in a thread-safe manner requires similar synchronization techniques as std::vector, so we can use boost::mutex to ensure thread safety when using boost::vector.

Example:

Output

Reading from vector: 
Writing to vector completed.
Reading from vector: 0 1 2 3 4 5 6 7 8 9 

Conclusion

Neither std::vector nor boost::vector is inherently thread-safe. To safely use these containers in a multi-threaded environment, we must use external synchronization mechanisms such as std::mutex or boost::mutex to ensure thread safety and to prevent data races for maintaining consistent states when accessing shared data structures concurrently.