Why can't I assign const int in struct constructor?

Question

Why can't I do this in C++?

struct SomeStruct
{
public:
    SomeStruct(const int someInt)
    {
        m_someInt = someInt;
    }

private:
    const int m_someInt;
};

Should the private field just be a regular integer?

`const` members can only be initialized in member initializer lists: https://en.cppreference.com/w/cpp/language/initializer_list (or using default values since C++11) — UnholySheep, Aug 11 '18 at 09:19
You can't assign to a `const int`, period. If you don't want your private field to be const, don't make it const. — n. 1.8e9-where's-my-share m., Aug 11 '18 at 09:27

score 9 · Accepted Answer · answered Aug 11 '18 at 09:22

You're assigning someInt to m_someInt, which is illegal. But initialization is okay.

struct SomeStruct
{
public:
    SomeStruct(const int someInt) : m_someInt(someInt)
    {

    }

private:
    const int m_someInt;
};

More info: Constructors and member initializer lists

Swift - Friday Pie · Answer 2 · 2018-08-11T10:13:39.800

Value cannot be assigned to a const storage. It can be only initialized. In case of class member variable it would be done in initialization list.

struct SomeStruct
{
public:
    SomeStruct(const int someInt) : m_someInt(someInt)
    {

    }

private:
    const int m_someInt;
};

Sometimes in-class initialization is enough:

template <int  Val>
struct SomeStruct
{
public:

private:
    const int m_someInt = Val;
};

Usually confusion stems from the fact that programmer doesn't see difference between two cases:

// 1) Declaring storage of int object that initially  contains value 5
int a = 5; // It's a declaration.

// 2) Declaring storage of int object that contains undetermined value
int b; // Declaration 
b = 5; // Assigning a value to it. It's a statement.

In your case m_someInt = someInt; is a statement that expects lvalue before =, and m_someInt is not a legal lvalue because it is const.

Why can't I assign const int in struct constructor?

2 Answers2