Setting std::string to 0 during definition vs setting std::string to 0

Question

I was working on one my projects, and while I was making the constructor for a class, I was setting some of my variables to a default value. I went to set an std::string to NULL, and it gave me an error. But when I define an std::string and set it to NULL on the same line, it works without any errors. I was wondering why

First Example

std::string text = NULL;

worked, and

Second Example

std::string text;
text = NULL;

didn't work. Now I know you shouldn't set a string to NULL, or 0, but I found this on accident.

Does the first example call a constructor that takes a char*, and thinking that 0 is a pointer to a char? I thought = called a constructor too, so I don't get why they wouldn't both work, unless std::string specifically overloads the = operator.

I am using Microsoft Visual Studio Express 2013

Aren't you getting run-time error, at line std::string text = NULL;? — 0x6773
– 0x6773, Commented Nov 14, 2015 at 7:45
@mnciitbhu nope, I am using Visual Studio 2013 Express. No errors at all. — Greg M
– Greg M, Commented Nov 14, 2015 at 7:52
@mnciitbhu so there's apparently some sort of discrepancy between the two — Greg M
– Greg M, Commented Nov 14, 2015 at 7:57

0x6773 · Accepted Answer · 2015-11-14 07:59:20Z

4

Value of NULL evaluates to 0.

Now, std::string text = NULL; will call constructor taking const char* and try to copy the data, but since NULL doesn't point to anything, an error will occur at run-time, I am getting (with gcc 5.2) :

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid

Since, there is no operator= defined for std::string and int(or size_t), text = NULL; will not compile.

edited Nov 14, 2015 at 7:59

answered Nov 14, 2015 at 7:31

0x6773

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10 Comments

Greg M Over a year ago

But doesn't the = call the size_t ctor again?

user4842163 Over a year ago

@GregM It calls the assignment operator. It's a bit confusing but using operator= in the context of initialization invokes an implicit constructor, while doing it post-initialization invokes an assignment operator.

user4842163 Over a year ago

It does, but where it gets hairy is implicit construction. For example, if you have implicit: Foo(int x), and no overloaded assignment operator for int, then trying to do: Foo f; f = 123; ... will actually call the parameterized constructor for Foo to implicitly construct it, and then the implicitly-generated assignment Foo& operator=(const Foo&);

DevSolar Over a year ago

@GregM: Basically, yes. Performance-wise, the compiler can do shortcuts as long as the semantics don't change (i.e. creating the "temporary" in-place, or -- since C++11 -- move-constructing it.) The idea is, since there is no way to assign an int to Foo, the compiler looks for alternatives, and finds that it's possible to assign a Foo to Foo and possible to construct a Foo from int, so that's what it does.

0x6773 Over a year ago

@GregM you can check if copy constructor is also used by deleting copy constructor, and checking if there are any errors. See ideone.com/nLke48

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bobobobo · Accepted Answer · 2021-10-09 01:19:03Z

1

The reason the 2nd assignment

text = NULL;

is in the VC++ compiler error message. It says:

error C2593: 'operator =' is ambiguous could be 'std::basic_string<char,std::char_traits,std::allocator> &std::basic_string<char,std::char_traits,std::allocator>::operator =(const _Elem)' or 'std::basic_string<char,std::char_traits,std::allocator> &std::basic_string<char,std::char_traits,std::allocator>::operator =(const _Elem *const )'

Breaking it down a bit, that says:

error C2593: 'operator =' is ambiguous could be 'std::string &std::string::operator =(const char)' or 'std::string &std::string::operator =(const char *const )'

So it doesn't work is because the compiler isn't sure whether 0 is meant to be a char value, or const char*.

It will compile if you use single quotes around the 0 to turn it into a char value:

text = '\0'; // NULL or can use any other character

Or if you cast it to a pointer type

type = (const char*)NULL; // Crashes, but now we're sure what type 0 is meant to be

that would compile also. Keep in mind assigning the std::string to a NULL pointer will crash as it tries to copy characters from NULL.

edited Oct 9, 2021 at 1:19

answered Oct 9, 2021 at 1:13

bobobobo

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2 Comments

Remy Lebeau Over a year ago

Along those lines, the reason the 1st constructor works is because there is no constructor that takes a single char, but there is one that takes a const char*, so passing NULL to the constructor is not ambiguous (just undefined behavior).

bobobobo Over a year ago

Yes, the std::string fill constructor that accepts char also requires an int argument, for how many of that char to use (it is std::string(int, char)

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