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Table 101 — Optional sequence container operations

 

Expression: a.emplace_back(args)

 

Return type: void

 

Operational semantics:
Appends an object of type T constructed with std::forward<Args>(args)....

Requires: T shall be EmplaceConstructible into X from args. For vector, T shall also be MoveInsertable into X.

§ 23.2.1.13

 

— T is EmplaceConstructible into X from args, for zero or more arguments args, means that the following expression is well-formed:

 

allocator_traits<A>::construct(m, p, args);

Table 101 — Optional sequence container operations

Expression: a.emplace_back(args)

Return type: void

Operational semantics:
Appends an object of type T constructed with std::forward<Args>(args)....

Requires: T shall be EmplaceConstructible into X from args. For vector, T shall also be MoveInsertable into X.

§ 23.2.1.13

— T is EmplaceConstructible into X from args, for zero or more arguments args, means that the following expression is well-formed:

allocator_traits<A>::construct(m, p, args);

For v.emplace_back({41,42});, see How do I use std::vector::emplace_back with vector<vector<int>>?

For v.emplace_back({41,42});, see How do I use std::vector::emplace_back with vector<vector<int>>?

v.emplace_back(41,42); does not work because of some rules in the standard (emphasis mine):

U(std::forward<Args>(args)...) (take note of the use of direct initialization) will find a constructor of U which accepts the forwarded arguments. However, in your case, my_pair is an aggregate type, which can only be initialized with braced-initialization syntax (aggregate initialization).

v.emplace_back(41,42); does not work because of some rules in the standard (some emphasis mine):

U(std::forward<Args>(args)...) (take note of the use of direct initialization) will find a constructor of U which accepts the forwarded arguments. However, in your case, my_pair is an aggregate type, which can only be initialized with braced-initialization syntax (aggregate initialization).