In this guide, you can learn how to use the C++ driver to retrieve an accurate and estimated count of the number of documents in a collection. The count_documents() method returns the exact number of documents that match a query filter or that exist in a collection, and the estimated_document_count() method returns the estimated number of documents in a collection.
The examples in this guide use the companies collection in the sample_training database from the Atlas sample datasets. To access this collection from your C++ application, instantiate a mongocxx::client that connects to an Atlas cluster and assign the following values to your db and collection variables:
auto db = client["sample_training"];auto collection = db["companies"];To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see the Get Started with Atlas guide.
Use the count_documents() method to count the number of documents that are in a collection. To count the number of documents that match a specific search critera, pass a query filter document to the count_documents() method.
To learn more about specifying a query, see the Specify a Query guide.
To return a count of all documents in the collection, pass an empty filter document to the count_documents() method, as shown in the following example:
auto result = collection.count_documents({});std::cout << "Number of documents: " << result << std::endl;Number of documents: 9500To return a count of documents that match specific search criteria, pass your query filter document to the count_documents() method.
The following example counts the number of documents that have a founded_year value of 2010:
auto result = collection.count_documents(make_document(kvp("founded_year", 2010)));std::cout << "Number of companies founded in 2010: " << result << std::endl;Number of companies founded in 2010: 33You can modify the behavior of the count_documents() method by passing an instance of the mongocxx::options::count class as a parameter. The following table describes the fields you can set in a mongocxx::options::count instance:
Field
Description
collation
The collation to use for the operation.
Type: bsoncxx::document::view_or_value
hint
The index to use for the operation.
Type: mongocxx::hint
comment
The comment to attach to the operation.
Type: bsoncxx::types::bson_value::view_or_value
limit
The maximum number of documents to count. This value must be a positive integer.
Type: std::int64_t
max_time
The maximum amount of time in milliseconds that the operation can run.
Type: std::chrono::milliseconds
skip
The number of documents to skip before counting documents.
Type: std::int64_t
read_preference
The read preference to use for the operation.
Type: mongocxx::read_preference
The following example uses the count_documents() method to count the number of documents in which the number_of_employees field has the value 50 and instructs the operation to count a maximum of 100 results:
mongocxx::options::count opts;opts.limit(100); auto result = collection.count_documents(make_document(kvp("number_of_employees", 50)), opts);std::cout << "Number of companies with 50 employees: " << result << std::endl;Number of companies with 50 employees: 100You can retrieve an estimate of the number of documents in a collection by calling the estimated_document_count() method. The method estimates the amount of documents based on collection metadata, which might be faster than performing an accurate count.
The following example estimates the number of documents in a collection:
auto result = collection.estimated_document_count();std::cout << "Estimated number of documents: " << result << std::endl;Estimated number of documents: 9500You can modify the behavior of the estimated_document_count() method by passing an instance of the mongocxx::options::estimated_document_count class as a parameter. The following table describes the fields you can set in a mongocxx::options::estimated_document_count instance:
Field
Description
max_time
The maximum amount of time in milliseconds that the operation can run.
Type: std::chrono::milliseconds
comment
The comment to attach to the operation.
Type: bsoncxx::types::bson_value::view_or_value
read_preference
The read preference to use for the operation.
Type: mongocxx::read_preference
The following example uses the estimated_document_count() method to return an estimate of the number of documents in the collection and instructs the operation to run for a maximum of 1000 milliseconds:
mongocxx::options::estimated_document_count opts;opts.max_time(std::chrono::milliseconds{1000}); auto result = collection.estimated_document_count(opts);std::cout << "Estimated number of documents: " << result << std::endl;Estimated number of documents: 9500To learn more about any of the methods or types discussed in this guide, see the following API documentation:
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