cppcoreguidelines-prefer-member-initializer

Finds member initializations in the constructor body which can be converted into member initializers of the constructor instead. This not only improves the readability of the code but also positively affects its performance. Class-member assignments inside a control statement or following the first control statement are ignored.

This check implements C.49 from the C++ Core Guidelines.

If the language version is C++ 11 or above, the constructor is the default constructor of the class, the field is not a bitfield (only in case of earlier language version than C++ 20), furthermore the assigned value is a literal, negated literal or enum constant then the preferred place of the initialization is at the class member declaration.

This latter rule is C.48 from the C++ Core Guidelines.

Please note, that this check does not enforce this latter rule for initializations already implemented as member initializers. For that purpose see check modernize-use-default-member-init.

Note

Enforcement of rule C.48 in this check is deprecated, to be removed in clang-tidy version 19 (only C.49 will be enforced by this check then). Please use cppcoreguidelines-use-default-member-init to enforce rule C.48.

Example 1

class C {
  int n;
  int m;
public:
  C() {
    n = 1; // Literal in default constructor
    if (dice())
      return;
    m = 1;
  }
};

Here n can be initialized using a default member initializer, unlike m, as m’s initialization follows a control statement (if):

class C {
  int n{1};
  int m;
public:
  C() {
    if (dice())
      return;
    m = 1;
  }
};

Example 2

class C {
  int n;
  int m;
public:
  C(int nn, int mm) {
    n = nn; // Neither default constructor nor literal
    if (dice())
      return;
    m = mm;
  }
};

Here n can be initialized in the constructor initialization list, unlike m, as m’s initialization follows a control statement (if):

C(int nn, int mm) : n(nn) {
  if (dice())
    return;
  m = mm;
}

UseAssignment

Note: this option is deprecated, to be removed in clang-tidy version 19. Please use the UseAssignment option from cppcoreguidelines-use-default-member-init instead.

If this option is set to true (by default UseAssignment from modernize-use-default-member-init will be used), the check will initialize members with an assignment. In this case the fix of the first example looks like this:

class C {
  int n = 1;
  int m;
public:
  C() {
    if (dice())
      return;
    m = 1;
  }
};