std::shared_lock::lock - cppreference.com (original) (raw)

| void lock(); | | (since C++14) | | ------------ | | ------------- |

Locks the associated mutex in shared mode. Effectively calls mutex()->lock_shared().

[edit] Parameters

(none)

[edit] Return value

(none)

[edit] Exceptions

• Any exceptions thrown by mutex()->lock_shared().
• If there is no associated mutex, std::system_error with an error code of std::errc::operation_not_permitted.
• If the associated mutex is already locked by this shared_lock (that is, owns_lock returns true), std::system_error with an error code of std::errc::resource_deadlock_would_occur.

[edit] Example

#include #include #include #include #include   std::string file = "Original content."; // Simulates a file std::mutex output_mutex; // mutex that protects output operations. std::shared_mutex file_mutex; // reader/writer mutex   void read_content(int id) { std::string content; { std::shared_lock lock(file_mutex, std::defer_lock); // Do not lock it first. lock.lock(); // Lock it here. content = file; } std::lock_guard lock(output_mutex); std::cout << "Contents read by reader #" << id << ": " << content << '\n'; }   void write_content() { { std::lock_guard file_lock(file_mutex); file = "New content"; } std::lock_guard output_lock(output_mutex); std::cout << "New content saved.\n"; }   int main() { std::cout << "Two readers reading from file.\n" << "A writer competes with them.\n"; std::thread reader1{read_content, 1}; std::thread reader2{read_content, 2}; std::thread writer{write_content}; reader1.join(); reader2.join(); writer.join(); std::cout << "The first few operations to file are done.\n"; reader1 = std::thread{read_content, 3}; reader1.join(); }

Possible output:

Two readers reading from file. A writer competes with them. Contents read by reader #1: Original content. Contents read by reader #2: Original content. New content saved. The first few operations to file are done. Contents read by reader #3: New content

[edit] See also

| | tries to lock the associated mutex (public member function) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | unlocks the associated mutex (public member function) [edit] |