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Compiler Error C3535

cannot deduce type for 'type1' from 'type2'

The type of the variable declared by the auto keyword cannot be deduced from the type of the initialization expression. For example, this error occurs if the initialization expression evaluates to void, which is not a type.

To correct this error

  1. Ensure that the type of the initialization expression is not void.

  2. Ensure that the declaration is not a pointer to a fundamental type. For more information, see Fundamental Types (C++).

  3. Ensure that if the declaration is a pointer to a type, the initialization expression is a pointer type.

The following example yields C3535 because the initialization expression evaluates to void.

// C3535a.cpp
// Compile with /Zc:auto
void f(){}
int main()
{
   auto x = f();   //C3535
   return 0;
}

The following example yields C3535 because the statement declares variable x as a pointer to a deduced type, but the type of the initializer expression is double. Consequently, the compiler cannot deduce the type of the variable.

// C3535b.cpp
// Compile with /Zc:auto
int main()
{
   auto* x = 123.0;   // C3535
   return 0;
}

The following example yields C3535 because variable p declares a pointer to a deduced type, but the initialization expression is not a pointer type.

// C3535c.cpp
// Compile with /Zc:auto
class A { };
A x;
auto *p = x;  // C3535
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