why using of explicit is causing compiler error

Question

I have seen many post regarding explicit keyword but no where find the answer for why its failing. I have seen other post in stack overflow also but Regarding the conversion its very confusing kind of explanation given

class Foo {
public:
  explicit Foo(int x);
};
int main()
{
Foo a = 42;         //Compile-time error 
Foo b(42);          //OK: calls Foo::Foo(int) passing 42 as an argument
}

what exactly happening when we are writing Foo a = 42; , and how its differ from Foo b(42);

Perhaps you could share the error with us? Do you know what explicit means for a constructor? — Retired Ninja, Mar 02 '18 at 04:11
error: conversion from 'int' to non-scalar type 'Foo' requested — sss, Mar 02 '18 at 04:12
Possible duplicate of [What does the explicit keyword mean?](https://stackoverflow.com/questions/121162/what-does-the-explicit-keyword-mean) — Retired Ninja, Mar 02 '18 at 04:14
Not duplicate, In that post there was no explanation for what asked — sss, Mar 02 '18 at 04:16
So you're saying that this answer https://stackoverflow.com/a/19250874/920069 is not exactly what you're asking about? — Retired Ninja, Mar 02 '18 at 04:18
stackoverflow.com/a/19250874/920069 , here its not explained of failure case. — sss, Mar 02 '18 at 04:27

score 0 · Answer 1 · answered Mar 02 '18 at 04:15

0

Foo a = 42;

if a constructor can be called with a single parameter, then it is a conversion constructor.Single argument can be converted to the class being constructed.

To disallow this you mark the constructor as explicit so such implicit conversions are not allowed, hence the error.

answered Mar 02 '18 at 04:15

Gaurav Sehgal

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if implicit conversions are not allowed then how its working for Foo b(42); – sss Mar 02 '18 at 04:24
`Foo b(42)` is not implicit constructor call. `Foo a = 42` is. – Gaurav Sehgal Mar 02 '18 at 04:25

score 0 · Answer 2 · answered Mar 02 '18 at 04:17

Foo a = 42; is copy initialisation.

And the C++ standard states that

explicit constructors are not converting constructors and are not considered for copy-initialization.

Reference: http://en.cppreference.com/w/cpp/language/copy_initialization

So it's a, perhaps lesser-known, effect of explicit.

why using of explicit is causing compiler error

2 Answers2