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mutable

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A specifier that can be applied to the declaration of a non-static, non-reference data member of a class. A mutable member of a class is not const even when the object is const. 

class C {
    int x;
    mutable int times_accessed;
  public:
    C(): x(0), times_accessed(0) {
    }
    int get_x() const {
        ++times_accessed; // ok: const member function can modify mutable data member
        return x;
    }
    void set_x(int x) {
        ++times_accessed;
        this->x = x;
    }
};

A second meaning for mutable was added in C++11. When it follows the parameter list of a lambda, it suppresses the implicit const on the lambda’s function call operator. Therefore, a mutable lambda can modify the values of entities captured by copy. See mutable lambdas for more details. 

std::vector<int> my_iota(int start, int count) {
    std::vector<int> result(count);
    std::generate(result.begin(), result.end(),
                  [start]() mutable { return start++; });
    return result;
}

Note that mutable is not a storage class specifier when used this way to form a mutable lambda.

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Table Of Contents
0 Getting started
1 Literals
2 Basic type keywords
3 Operator precedence
4 Floating point arithmetic
5 Bit operators
6 Bit manipulation
7 Bit fields
8 Arrays
9 Flow control
10 const keyword
11 Loops
12 mutable keyword
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102 Attributes
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136 Debugging
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138 More undefined behaviors
139 Mutexes
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141 Unit testing
142 decltype
143 Digit separators
144 C++ Containers
145 Arithmetic meta-programming
146 Contributors