Calling Deleted Function: Error with Using std::unique_ptr

Introduction

During software development, developers may encounter situations where the compiler issues an error related to calling a deleted function. In this article, we will examine the problem of using std::unique_ptr, which leads to a compilation error, and provide solutions for its resolution.

The Problem and Analysis

Consider the following code:

#include <map>
#include <vector>
#include <memory>

struct ValidatorSet {};

class Field {
public:
    struct ValidationProperties {
        std::map<int, std::vector<std::unique_ptr<ValidatorSet>>> m_validators;
    };

    ValidationProperties GetFieldProperties() const {
        return ValidationProperties{};
    }
};

class SQLiteDatabase {
public:
    virtual int GetFieldProperties(const std::wstring&, const std::wstring&, const std::wstring&, const std::wstring&, TableField*, std::vector<std::wstring>&) {
        auto field = new Field();
        auto valid : field->GetFieldProperties().m_validations.m_validators[type];
        valid.reset(); // Compilation error here
        return 0;
    }
};

Attempting to compile this code results in the following error:

error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = std::tuple<std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>>; _Dp = std::default_delete<std::tuple<std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>>>]'

This error indicates that the compiler does not allow creating a copy of std::unique_ptr because the copy constructor has been deleted (= delete).

Cause of the Error

The std::unique_ptr class does not have a default copy constructor. This is done to avoid non-intuitive and potentially dangerous situations related to memory management. If a copy constructor were available, it could lead to resource management issues, especially in a multithreaded context.

Solution to the Problem

To fix this error, you need to use methods like reset() or swap() to manage pointers. For example, you can use swap() to exchange the contents of two std::unique_ptr objects.

Using swap()

auto& validators = field->GetFieldProperties().m_validations.m_validators[type];
validators.swap({});

Using reset()

auto& validators = field->GetFieldProperties().m_validations.m_validators[type];
validators.reset();

Both methods allow safe resource release without creating a copy.

Practical Tips

1. Avoid Copying std::unique_ptr: Always use swap() or reset() to manage pointers.
2. Use Smart Pointers: std::unique_ptr and std::shared_ptr provide convenient mechanisms for managing memory, avoiding leaks and resource management issues.
3. Check Identifiers: Ensure you correctly use identifiers and indices when accessing collections.

Conclusion

The problem of calling a deleted function with std::unique_ptr can be complex to understand and resolve. However, using appropriate pointer management methods and avoiding copying std::unique_ptr will help prevent such errors. In this guide, we examined the cause of the error and provided solutions for its resolution.