String find function in C++ returns 0 if string passed as argument is empty

Question

When a empty string is passed to find function it returns 0.
If uninitialized string is passed, then also it returns 0.

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str1="";
    string str2="This is a sample string";
    unsigned int loc = str2.find(str1);
    cout << "Loc : " << loc << endl;
    if(loc != string::npos)
    {
        cout << "Found" << endl;
    }
    else
    {
        cout << "Not found" << endl;
    }
    return 0;
}

Output :
Loc : 0
Found

My question is why does find return 0 instead of returning string::npos ?

for the records, the empty string is equivalent to an uninitialized one. — Guillaume Racicot
– Guillaume Racicot, Commented Feb 2, 2021 at 17:49
after string str1; str1 is initialized; the default constructor is used to initialize the string which happens to initialize it as the empty string. It's rather hard to get an uninitialized string. you'll need to jump some hoops to get an uninitialized string: void* memory = std::malloc(sizeof(std::string)); std::string* uninitializedStringPointer = static_cast<std::string*>(memory); — fabian
– fabian, Commented Feb 2, 2021 at 20:22

orlp · Accepted Answer · 2021-02-02 17:51:27Z

5

A needle l can be found in position i of haystack h if i + len(l) <= len(h) and for all 0 <= k < len(l) we have l[k] == h[i+k]. What you're dealing with is simply the vacuous truth of this definition: 'for all x' automatically becomes true if there's no valid x to begin with.

answered Feb 2, 2021 at 17:51

orlp

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Comments

Werner Henze · Accepted Answer · 2021-02-02 17:54:58Z

2

As the docs on cppreference say. (Emphasis is mine.)

Finds the first substring equal to the given character sequence. Search begins at pos, i.e. the found substring must not begin in a position preceding pos.

Finds the first substring equal to str.

When is a string found at a position?

Formally, a substring str is said to be found at position xpos if all of the following is true:

xpos >= pos

xpos + str.size() <= size()

for all positions n in str, Traits::eq(at(xpos+n), str.at(n))

An empty string fulfills these requirements at position 0.

The docs further say. (Emphasis is mine.)

In particular, this implies that

a substring can be found only if pos <= size() - str.size()

an empty substring is found at pos if and only if pos <= size()

for a non-empty substring, if pos >= size(), the function always returns npos.

edited Feb 2, 2021 at 17:54

answered Feb 2, 2021 at 17:52

Werner Henze

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2 Comments

463035818_is_not_an_ai Over a year ago

it isnt mentioned explicitly, though "for all positions n in str, Traits::eq(at(xpos+n), str.at(n))" implies that an empty substring can by found anywhere, because its size is 0, hence anything is true for any position in the substring

Werner Henze Over a year ago

@largest_prime_is_463035818 The docs say that find finds the first substring.

Yksisarvinen · Accepted Answer · 2021-02-02 17:55:47Z

1

It found the exact match at position 0 (the first one searched), so that index is returned.

Quoting from cppreference, the conditions are:

Formally, a substring str is said to be found at position xpos if all of the following is true:

xpos >= pos

xpos + str.size() <= size()

for all positions n in str, Traits::eq(at(xpos+n), str.at(n))

Trying to match substring from position 0, we get that

is met, because 0 (our search position) >= 0 (start parameter)
is met, because 0 + 0 <= size()
is met, because there is no character in the needle that doesn't match the corresponding character in haystack - all of them match.

answered Feb 2, 2021 at 17:55

Yksisarvinen

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