std:: max

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max
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Defined in header <algorithm>
template < class T >
const T & max ( const T & a, const T & b ) ;
(1) (constexpr since C++14)
template < class T, class Compare >
const T & max ( const T & a, const T & b, Compare comp ) ;
(2) (constexpr since C++14)
template < class T >
T max ( std:: initializer_list < T > ilist ) ;
(3) (since C++11)
(constexpr since C++14)
template < class T, class Compare >
T max ( std:: initializer_list < T > ilist, Compare comp ) ;
(4) (since C++11)
(constexpr since C++14)

Returns the greater of the given values.

1,2) Returns the greater of a and b .
1) Uses operator < to compare the values.
If T is not LessThanComparable , the behavior is undefined.
2) Use the comparison function comp to compare the values.
3,4) Returns the greatest of the values in initializer list ilist .
If ilist. size ( ) is zero, or T is not CopyConstructible , the behavior is undefined.
3) Uses operator < to compare the values.
If T is not LessThanComparable , the behavior is undefined.
4) Use the comparison function comp to compare the values.

Parameters

a, b - the values to compare
ilist - initializer list with the values to compare
comp - comparison function object (i.e. an object that satisfies the requirements of Compare ) which returns true if a is less than b .

The signature of the comparison function should be equivalent to the following:

bool cmp ( const Type1 & a, const Type2 & b ) ;

While the signature does not need to have const & , the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed , nor is Type1 unless for Type1 a move is equivalent to a copy (since C++11) ).
The types Type1 and Type2 must be such that an object of type T can be implicitly converted to both of them.

Return value

1,2) The greater of a and b . If they are equivalent, returns a .
3,4) The greatest value in ilist . If several values are equivalent to the greatest, returns the leftmost one.

Complexity

1) Exactly one comparison using operator < .
2) Exactly one application of the comparison function comp .
3,4) Given N as ilist. size ( ) :
3) Exactly N-1 comparisons using operator < .
4) Exactly N-1 applications of the comparison function comp .

Possible implementation

max (1) 
template<class T> 
const T& max(const T& a, const T& b)
{
    return (a < b) ? b : a;
}
max (2) 
template<class T, class Compare> 
const T& max(const T& a, const T& b, Compare comp)
{
    return (comp(a, b)) ? b : a;
}
max (3) 
template<class T>
T max(std::initializer_list<T> ilist)
{
    return *std::max_element(ilist.begin(), ilist.end());
}
max (4) 
template<class T, class Compare>
T max(std::initializer_list<T> ilist, Compare comp)
{
    return *std::max_element(ilist.begin(), ilist.end(), comp);
}

Notes

Capturing the result of std::max by reference produces a dangling reference if one of the parameters is a temporary and that parameter is returned: 

int n = -1;
const int& r = std::max(n + 2, n * 2); // r is dangling

Example

Run this code 
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <string_view>
 
int main()
{
    auto longest = [](const std::string_view s1, const std::string_view s2)
                   {
                       return s1.size() < s2.size();
                   };
 
    std::cout << "Larger of 69 and 96 is " << std::max(69, 96) << "\n"
                 "Larger of 'q' and 'p' is '" << std::max('q', 'p') << "'\n"
                 "Largest of 010, 10, 0X10, and 0B10 is "
              << std::max({010, 10, 0X10, 0B10}) << '\n'
              << R"(Longest of "long", "short", and "int" is )"
              << std::quoted(std::max({"long", "short", "int"}, longest)) << '\n';
}

Output: 

Larger of 69 and 96 is 96
Larger of 'q' and 'p' is 'q'
Largest of 010, 10, 0X10, and 0B10 is 16
Longest of "long", "short", and "int" is "short"

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 281 C++98 T was required to be CopyConstructible for overloads (1,2) not required

See also

returns the smaller of the given values
(function template)
(C++11)
returns the smaller and larger of two elements
(function template)
returns the largest element in a range
(function template)
(C++17)
clamps a value between a pair of boundary values
(function template)
(C++20)
returns the greater of the given values
(algorithm function object)
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