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Expensive use of container's insertion method

One of the insertion methods of a container is used to insert a temporary object

Since R2022a

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Description

This defect occurs when you use a container's insertion method to insert a temporary object into a container. For instance:

std::vector<std::string> v;
v.push_back("foo");//Defect
This checker flags use of these methods to insert a temporary object:

  • insert()

  • push()

  • push_front()

  • push_back()

Most containers from the C++ standard template library implements these insertion methods and are supported by this checker.

Risk

When you insert a temporary object by using a container's insertion method, the compiler constructs a temporary object, and then moves or copies the object into the container. Because this insertion method requires a move or copy operation in addition to a construction, it is less efficient. Using this method results in inefficient code.

Fix

Avoid the unnecessary move or copy operation by constructing the object-to-be-inserted in place in the container by using the container's emplace methods. Such methods might include emplace(), emplace_front(), or emplace_back(). Because these methods require no copy or move operations, they are more efficient.

When using emplace methods, avoid supplying constructor functions manually. It is more efficient to allow the emplace methods to construct the object. For instance:

std::vector<std::string> v;
v.emplace_back(std::string("foo"));//Compliant but less efficient
v.emplace_back("foo");// Efficient fix
 std::map<int, std::string> m;
m.emplace(std::make_pair(5, "foo"));//Compliant but less efficient 
m.emplace(5, "foo");// Efficient fix
Using v.emplace_back(std::string("foo")) or m.emplace(std::make_pair(5, "foo")) does not raise a violation of this checker. But it is more efficient to use emplace methods without specifying constructor functions, such as m.emplace(5, "foo").

Performance improvements might vary based on the compiler, library implementation, and environment that you are using.

Examples

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#include <vector>
#include <string>

void addNameToList(std::vector<std::string> &names, const char *pszName)
{
    names.push_back(pszName); //Defect
}

In this example, the method push_back is used to insert a temporary string into a vector. Using the push_back method necessitates constructing a string object, and then moving the object into the vector. The move operation results in inefficient code. Polyspace® flags the method push_back.

Correction — Construct Object In Place

To resolve this defect, construct the object to be inserted in place by using emplace_back. This method avoids the unnecessary move operation, resulting in more efficient code.

#include <vector>
#include <string>

void addNameToList(std::vector<std::string> &names, const char *pszName)
{
    names.emplace_back(pszName);
}

Result Information

Group: Performance
Language: C++
Default: Off
Command-Line Syntax: EXPENSIVE_CONTAINER_INSERTION
Impact: Medium

Version History

Introduced in R2022a

See Also

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